CN112567061B

CN112567061B - Steel material, forged heat-treated product, and method for producing forged heat-treated product

Info

Publication number: CN112567061B
Application number: CN201980054023.1A
Authority: CN
Inventors: 宫越有祐; 高须贺幹
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2018-08-24
Filing date: 2019-08-21
Publication date: 2022-12-16
Anticipated expiration: 2039-08-21
Also published as: JP7063386B2; JPWO2020040190A1; WO2020040190A1; CN112567061A

Abstract

Has high machinability, yield strength and fatigue strength and obtains excellent cracking properties. The steel material has the following chemical composition: c in mass%: 0.10 to 0.60%, si:0.05 to 1.00%, mn:0.30 to 1.50%, P:0.1000% or less, S:0.3000% or less, al:0.003 to 0.100%, N:0.0200% or less, and the balance: fe and impurities, and satisfies formula (1). More than 70.0% by mass of Al ₂ O ₃ And the number density of inclusions having an AREA of 3 μm or more is 0.05 to 1.00 inclusions/mm ² .9 is less than or equal to 7.6 √ C × (1 + 0.6Si) × (1 +4 Mn) × (1-0.6S) × (1 + 0.3Cu) × (1 + 0.5Ni) × (1 + 2Cr) × (1 + 3Mo) × (1 + (1.5 × (0.9-C) × fB)) < 130 (1). In the formula (1), fB is "0" when the B content (mass%) is 0%, and "1" when it exceeds 0%.

Description

Steel material, forged heat-treated product, and method for producing forged heat-treated product

Technical Field

The present invention relates to a steel material, a forged heat-treated product produced using the steel material, and a method for producing the forged heat-treated product.

Background

A connecting rod (hereinafter also referred to as "connecting rod") used in an automobile engine or the like is an engine component that connects a piston and a crankshaft, and converts reciprocating motion of the piston into rotational motion of a crank.

Fig. 1 is a front view of a conventional link. As shown in fig. 1, the conventional link 1 includes a large head portion 100, a shaft portion 200, and a small head portion 300. The large head 100 is disposed at one end of the shaft portion 200, and the small head 300 is disposed at the other end of the shaft portion 200. The large head 100 is coupled to the crank pin. Small head 300 is attached to the piston.

The conventional link 1 includes two parts (a cover 2 and a lever 3). These parts are typically manufactured by hot forging. One end of the cover 2 and the rod 3 corresponds to the large end 100. The other portions than one end of the shaft 3 correspond to the shaft portion 200 and the small head portion 300. The large head portion 100 and the small head portion 300 are formed by cutting. Therefore, the connecting rod 1 is required to have high machinability.

The connecting rod 1 receives loads from surrounding members when the engine is running. Recently, in order to further reduce fuel consumption, downsizing of the connecting rod 1 and increase of the in-cylinder pressure in the cylinder have been demanded. Therefore, the connecting rod 1 is required to have an excellent yield strength capable of coping with the explosion load transmitted from the piston even if the shaft body portion 200 is narrowed. Further, since the connecting rod 1 is subjected to repeated compression load and tensile load, excellent fatigue strength is also required.

However, as described above, the conventional link 1 is formed by separately manufacturing the cover 2 and the lever 3. Therefore, a positioning pin processing step is performed for positioning the cover 2 and the rod 3. Further, a cutting process is performed on the joint surface between the cap 2 and the rod 3. Therefore, a fracture rod capable of omitting these steps has begun to be widespread.

In the split link, after the link is integrally molded, a jig is inserted into a hole of the large head 100, and the large head is broken by a load stress to be divided into two parts (corresponding to the cap 2 and the rod 3). Then, the divided two parts are joined together when the crankshaft is attached. If the fracture surface of the large head 100 is a brittle fracture surface without deformation, the cap 2 and the fracture surface of the rod 3 may be joined together by bolts. Therefore, in this case, the positioning pin machining step and the cutting machining step are omitted. As a result, the manufacturing cost is reduced.

The split connecting rod is generally formed integrally by hot forging. In the present specification, a product obtained by hot forging a steel material and then heat-treating the hot forged product is also referred to as a "forged heat-treated product". Here, in the case of being used for a fracture splitting connecting rod, the toughness of the forged heat-treated product is preferably low. In the case of a steel having high toughness, when a large head is fractured by cracking, a ductile fracture surface is easily generated at the fracture surface. At this time, the large head portion is plastically deformed. Therefore, even if the fracture surfaces are merged, they will not perfectly match, and the inner diameter D of the large head 100 in fig. 1 may deviate from the desired value. As a result, the crank connecting portion (large head portion) may partially come into contact with each other, which may cause vibration and noise during traveling of the automobile.

As a result of hot forging using a steel material having an adjusted chemical composition in order to improve the yield strength and fatigue strength of a forged heat-treated product which is required to have high cracking properties, if the sum total of the area ratios of tempered martensite and tempered bainite in the structure of the steel material (forged heat-treated product) obtained by heat treatment after hot forging is 80% or more, the machinability of the steel material is further reduced, and the cutting resistance at the time of drilling a bolt hole is increased. If the cutting resistance during drilling increases, the tool life decreases or the load on the driving member in the cutting machine increases. Therefore, in the case of improving the yield strength and fatigue strength of the forged heat-treated product, it is required to further improve the machinability (suppress the cutting resistance) of the steel material in the production process of the forged heat-treated product.

Jp 2004-277817 a (patent document 1), jp 2011-195862 a (patent document 2), international publication No. 2009/107282 (patent document 3), jp 2006-336071 a (patent document 4), jp 2016-27204 a (patent document 5), and jp 2017-106099 a (patent document 6) propose steels having high cracking properties.

The composition of the high-strength non-heat-treated steel disclosed in patent document 1 is as follows: c in weight percent: 0.2 to 0.6%, si:0.1 to 2%, mn:0.1 to 1.5%, S:0.03 to 0.2%, P:0.02 to 0.15%, cu:0.03 to 1%, ni:0.03 to 1%, cr:0.05 to 1%, V:0.02 to 0.4%, ti:0.01 to 0.8%, s-Al: 0.005-0.045%, N:0.008 to 0.035%, and the balance of unavoidable impurities and Fe, and has a ferrite pearlite structure. The maximum diameter of TiN inclusions in the steel is 5 μm or more, and the amount thereof is 5/mm in number density ² As described above. Patent document 1 describes that the non-heat-treated steel has high strength and good machinability, and is excellent in fracture separation performance and capable of forming good irregularities on a fracture surface.

The non-heat-treated steel for hot forging disclosed in patent document 2 contains, in mass%, C:0.35 to 0.55%, si:0.15 to 0.40%, mn:0.50 to 1.00%, P:0.100% or less, S:0.040 to 0.100%, cr:1.00% or less, V:0.20 to 0.50%, ca:0.0005 to 0.0100%, N:0.0150% or less, and the balance of Fe and inevitable impurities. The chemical composition of the steel satisfies the conditions that 2Mn +5Mo + Cr is less than or equal to 3.1, C + Si/5+ Mn/10+10P +5V is more than or equal to 1.8, and Ceq = C + Si/7+ Mn/5+ Cr/9+ V is 0.90-1.10. The steel has a hardness of HV330 or more and a yield ratio of 0.73 or more. The steel has a ferrite/pearlite structure with bainite of 10% or less. Patent document 2 describes that the hot forging non heat treated steel can provide a hot forging non heat treated steel member capable of ensuring high strength and ensuring excellent machinability and fracture separability.

The non-heat-treated steel for hot forging disclosed in patent document 3 contains, in mass%, C: more than 0.35% and 0.60% or less, si:0.50 to 2.50%, mn:0.20 to 2.00%, P:0.010 to 0.150%, S:0.040 to 0.150%, V:0.10 to 0.50%, zr:0.0005 to 0.0050%, ca:0.0005 to 0.0050%, N:0.0020 to 0.0200%, al is limited to less than 0.010%, and the balance is substantially Fe and unavoidable impurities. Patent document 3 describes that the non-heat-treated steel for hot forging is excellent in fracture splittability and machinability.

The steel for a connecting rod disclosed in patent document 4 contains, in mass%, C:0.1 to 0.5%, si:0.1 to 2%, mn:0.5 to 2%, P:0.15% or less (excluding 0%), S:0.06 to 0.2%, N:0.02% or less (excluding 0%), ca:0.0001 to 0.005% and Al:0.001 to 0.02%, and the balance of Fe and unavoidable impurities. Patent document 4 describes that the composition of oxide inclusions in the steel for a connecting rod is controlled within a predetermined range, and therefore, the fracture splittability can be improved.

The age-hardening bainite non-heat-treated steel disclosed in patent document 5 contains, in mass%, C:0.10 to 0.40%, si:0.01 to 2.00%, mn:0.10 to 3.00%, P:0.001 to 0.150%, S:0.001 to 0.200%, cu:0.001 to 2.00%, ni:0.40% or less, cr:0.10 to 3.00%, further comprising Mo:0.02 to 2.00%, V:0.02 to 2.00%, ti:0.001 to 0.250%, nb:0.01 to 0.10% of any 1 or 2 or more, and the balance being Fe and unavoidable impurities, and the content mass% of the specified chemical components satisfying 3 x [ C ] +10 x [ Mn ] +2 x [ Cu ] +2 x [ Ni ] +12 x [ Cr ] +9 x [ Mo ] +2 x [ V ] ≧ 20, 32 x [ C ] +3 x [ Si ] +3 x [ Mn ] +2 x [ Ni ] +3 x [ Cr ] +11 x [ Mo ] +32 x [ V ] +65 x [ Ti ] +36 x [ Nb ] ≧ 24, 321 x [ C ] -31 x [ Mo ] +213 x [ V ] +545 +11 x [ Mo ] +280 x [ Nb ] + 100, 321 x [ C ] -31 x [ Mo ] +280 x [ V ] + 545. Patent document 5 describes that plastic deformation during fracture splitting can be satisfactorily suppressed even in a member produced by fracture splitting.

The steel for fracture-splitting type connecting rods disclosed in patent document 6 contains, in mass%, C:0.01 to 0.5%, si: more than 0% and 0.7% or less, mn:0.01 to 3%, P:0.001 to 0.2%, S: more than 0% and 0.2% or less, cr:0.01 to 3%, al: more than 0% and 0.1% or less, and N: more than 0% and not more than 0.03%, and the balance of iron and inevitable impurities, represented by the formula (DI =1.16 × ([ C))]/10) ^1/2 ×(0.7×[Si]+1)×〔5.1×{[Mn]-1.2-(55/32×[S])}+5〕×(0.35×[Cu]+1)×(0.36×[Ni]+1)×(2.16×[Cr]+1)×(3×[Mo]+ 1) × 25.4) is 55 to 200mm, and the sum of the area fractions of tempered martensite and bainite is 80 area% or more with respect to the entire metallographic structure. Patent document 6 describes that the steel for fracture-split type tie rods is strongOn the basis of the improved degree, no quench cracking occurs during quenching, and the fracture separability can be improved.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2004-277817

Patent document 2: japanese patent laid-open publication No. 2011-195862

Patent document 3: international publication No. 2009/107282

Patent document 4: japanese patent laid-open publication No. 2006-336071

Patent document 5: japanese patent laid-open publication No. 2016-27204

Patent document 6: japanese patent laid-open publication No. 2017-106099

Disclosure of Invention

Problems to be solved by the invention

However, patent documents 1 to 5 are premised on that the microstructure of the steel material after hot forging mainly contains any one or more of ferrite, pearlite, and bainite. Therefore, when the total area ratio of tempered martensite and tempered bainite is 80% or more in the microstructure of the forged heat-treated product, the cracking property of the forged heat-treated product may be lowered.

In patent document 6, the total of the area ratios of tempered martensite and tempered bainite in the microstructure of a forged heat-treated product is 80% or more. However, by a method different from the steel for fracture-split type connecting rods disclosed in patent document 6, it is possible to have high machinability, high yield strength, and high fatigue strength, and in the microstructure after hot forging, even if the sum of the area fractions of tempered martensite and tempered bainite is 80% or more, excellent fracture properties can be obtained.

An object of the present invention is to provide a steel material having excellent hot workability, which, when a forged heat-treated product is produced by subjecting the steel material to heat treatment after hot forging, has excellent machinability, high yield strength and high fatigue strength after hot forging and heat treatment, and has excellent cracking properties after hot forging and heat treatment even when the sum of the area fractions of tempered martensite and tempered bainite in the microstructure of the forged heat-treated product reaches 80% or more.

Means for solving the problems

The steel based on the application has the following chemical composition: in mass percent

C：0.10～0.60％、

Si：0.05～1.00％、

Mn：0.30～1.50％、

P: less than 0.1000 percent,

S: less than 0.3000 percent,

Al：0.003～0.100％、

N: less than 0.0200%,

Cr：0～2.50％、

Cu：0～0.60％、

Ni：0～0.60％、

Mo：0～0.70％、

V：0～0.049％、

Ti：0～0.250％、

B：0～0.0050％、

Nb：0～0.100％、

Te：0～0.3000％、

Ca：0～0.0100％、

Bi:0 to 0.4000%, and

the balance is as follows: fe and impurities in the iron-based alloy, wherein the impurities are,

the Cr content is 0-0.50%, the formula (1) is satisfied,

satisfying the formula (2) when the Cr content is 0.51-2.50%;

will contain more than 70.0% by mass of Al ₂ O ₃ And inclusions having an AREA of 3 μm or more are defined as coarse Al ₂ O ₃ When the foreign matter is included, the mixture is mixed,

the coarse Al in the steel material ₂ O ₃ The number density of the inclusions is 0.05 to 1.00 pieces/mm ² 。

9≤7.6√C×(1+0.6Si)×(1+4Mn)×(1-0.6S)×(1+0.3Cu)×(1+0.5Ni)×(1+2Cr)×(1+3Mo)×(1+(1.5×(0.9-C)×fB))≤130 (1)

40≤7.6√C×(1+0.6Si)×(1+4Mn)×(1-0.6S)×(1+0.3Cu)×(1+0.5Ni)×(1+2Cr)×(1+3Mo)×(1+(1.5×(0.9-C)×fB))≤300 (2)

Here, the content (mass%) of the corresponding element is substituted at the symbol of the element in the formula (1) and the formula (2). In addition, when no corresponding element is contained, the symbol of the element is substituted by "0". fB in the formulae (1) and (2) is "0" when the B content (mass%) is 0% and "1" when the B content (mass%) exceeds 0%.

The forged heat-treated product based on the present application has the following chemical composition: in mass%)

C：0.10～0.60％、

Si：0.05～1.00％、

Mn：0.30～1.50％、

P: less than 0.1000 percent,

S: less than 0.3000%,

Al：0.003～0.100％、

N: less than 0.0200%,

Cr：0～2.50％、

Cu：0～0.60％、

Ni：0～0.60％、

Mo：0～0.70％、

V：0～0.049％、

Ti：0～0.250％、

B：0～0.0050％、

Nb：0～0.100％、

Te：0～0.3000％、

Ca：0～0.0100％、

Bi:0 to 0.4000%, and

and the balance: fe and impurities in the iron-based alloy, and the impurities,

satisfying the formula (1) when the Cr content is 0-0.50%,

the Cr content is 0.51-2.50%, and the formula (2) is satisfied;

more than 70.0% of Al is contained in mass% ₂ O ₃ And inclusions having an AREA of 3 μm or more are defined as coarse Al ₂ O ₃ When the foreign matter is included, the mixture is mixed,

the coarse Al contained in the forged heat-treated product ₂ O ₃ The number density of the inclusions is 0.05 to 1.00 pieces/mm ² ；

The microstructure of the forged heat-treated product has a total area ratio of tempered martensite to tempered bainite of 80% or more.

The method for producing a forged heat-treated product according to the present application includes the steps of:

a hot forging step of heating the steel material to 1100 to 1300 ℃ and performing hot forging to produce an intermediate product;

a quenching step of cooling the intermediate product after the hot forging step at an average cooling rate of 10 to 200 ℃/sec at 800 to 100 ℃; and

a tempering step of holding the intermediate product at 400 to 650 ℃ for 30 to 90 minutes after the quenching step.

ADVANTAGEOUS EFFECTS OF INVENTION

The steel material according to the present application has excellent hot workability, and when a forged heat-treated product is produced by subjecting the steel material to heat treatment after hot forging, the forged heat-treated product has excellent machinability, high yield strength, and high fatigue strength after hot forging and heat treatment, and has excellent cracking properties after hot forging and heat treatment even when the sum of the area fractions of tempered martensite and tempered bainite in the microstructure of the forged heat-treated product reaches 80% or more. The forged heat-treated product based on the present application has excellent machinability, high yield strength, high fatigue strength, and excellent cracking properties. The method for producing a forged heat-treated product according to the present application can produce the forged heat-treated product from the steel material.

Drawings

Fig. 1 is a front view of a conventional tie rod.

FIG. 2A is a plan view of a test piece used in the cleavage evaluation test in the examples.

FIG. 2B is a cross-sectional view of the test piece shown in FIG. 2A.

Fig. 2C is a plan view of the test piece in the state where the test piece of fig. 2A is fractured and separated.

Fig. 2D is a plan view of the test piece in the state where the test piece of fig. 2C is fastened with a bolt.

Detailed Description

Hereinafter, embodiments of the present invention will be described in detail.

The present inventors investigated and studied hot workability in a process for producing a forged heat-treated product using a steel material, and yield strength, fatigue strength, machinability, and cracking properties of a forged heat-treated product produced using a steel material. As a result, the present inventors have obtained the following findings.

(A) With respect to yield strength and fatigue strength

As a result of the hot forging and heat treatment performed on the steel material of the present embodiment, if the total area ratio of tempered martensite and tempered bainite in the microstructure becomes 80% or more, the yield strength and fatigue strength of the steel material (forged heat-treated product) after the hot forging and heat treatment are improved. That is, the steel material of the present embodiment is required to have excellent hardenability. First, the present inventors considered that: if C is prepared in mass percent: 0.10 to 0.60%, si:0.05 to 1.00%, mn:0.30 to 1.50%, P:0.1000% or less, S:0.3000% or less, al:0.003 to 0.100%, N:0.0200% or less, cr:0 to 2.50%, cu:0 to 0.60%, ni:0 to 0.60%, mo:0 to 0.70%, V:0 to 0.049%, ti:0 to 0.250%, B:0 to 0.0050%, nb:0 to 0.100%, te:0 to 0.3000%, ca:0 to 0.0100%, bi:0 to 0.4000%, and the balance: the chemical composition of Fe and impurities may improve the hardenability of the steel.

On the other hand, the present inventors have found that: in the steel material having the above chemical composition, the alloying element may be locally segregated. That is, as a result of increasing the content of the alloying element to make the chemical composition described above in order to improve the hardenability of the steel material, segregation of the alloying element may occur in the steel material. When the alloy element is segregated in the steel, the melting point of the segregated portion is lower than that of the base material. As a result, the segregation portion is melted and then solidified in the form of an oxide or the like during heating in hot forging. At this time, oxides and the like in the forged heat-treated product may become starting points of fatigue fracture.

However, if the content of the alloying element is reduced in order to reduce segregation in the steel material, the steel material may not have excellent hardenability. Therefore, in the steel material having the above chemical composition, the content of the alloying element may be adjusted in order to achieve both hardenability and reduction of segregation.

Specifically, for a steel material having the above chemical composition, it is defined as fn1=7.6 √ C × (1 +0.6 Si) × (1 +4 Mn) × (1-0.6S) × (1 +0.3 Cu) × (1 + 0.5Ni) × (1 + 2Cr) × (1 + 3Mo) × (1 + (1.5 × (0.9-C) × fB)). fn1 is an index of hardenability and segregation in the steel material having the above chemical composition. Here, the content (% by mass) of the corresponding element is substituted into the symbol of the element in fn 1. In addition, when no corresponding element is contained, the symbol of the element is substituted by "0". fB in fn1 is "0" when the B content (mass%) is 0%, and "1" when the B content (mass%) exceeds 0%.

When the Cr content is 0 to 0.50%, if fn1 is higher than 130, segregation of alloying elements occurs in the steel. At this time, the fatigue strength of the steel material (forged heat-treated product) after the hot forging and heat treatment is lowered. If fn1 is less than 9, the steel material cannot sufficiently obtain hardenability. In this case, the sum of the area fractions of tempered martensite and tempered bainite in the microstructure of the forged heat-treated product is less than 80%, and the yield strength and/or fatigue strength of the forged heat-treated product is lowered. Therefore, in the case where the Cr content is 0 to 0.50%, if fn1 is 9 to 130, the forged heat-treated product produced from the steel material having the above chemical composition can obtain excellent fatigue strength and yield strength.

When the Cr content is 0.51 to 2.50%, if fn1 is higher than 300, segregation of alloying elements occurs in the steel. At this time, the fatigue strength of the steel material (forged heat-treated product) after the hot forging and heat treatment is lowered. If fn1 is less than 40, the steel material cannot sufficiently obtain hardenability. In this case, the sum of the area fractions of tempered martensite and tempered bainite in the microstructure of the forged heat-treated product is less than 80%, and the yield strength and/or fatigue strength of the forged heat-treated product is lowered. Therefore, in the case where the Cr content is 0.51 to 2.50%, if fn1 is 40 to 300, the forged heat-treated product produced from the steel material having the above chemical composition can obtain excellent fatigue strength and yield strength.

(B) With respect to the cracking property

As described above, as a result of hot forging and heat treatment of a steel material as a raw material for the purpose of improving the yield strength and fatigue strength of a forging heat-treated product requiring high cracking properties, it is assumed that cracking properties are lowered when the total area ratio of tempered martensite and tempered bainite reaches 80% or more in a microstructure of the steel material (forging heat-treated product) after hot forging and heat treatment. This is because: tempered martensite and tempered bainite have high toughness, and a ductile fracture surface is easily formed in a fracture surface after cracking. Therefore, when improving the cracking property of the steel material, the microstructure is preferably mainly ferrite and pearlite.

However, tempered martensite and/or tempered bainite increase the fatigue strength, yield strength, of the steel. Therefore, if a technique is available in which the sum of the area ratios of tempered martensite and tempered bainite in the microstructure of the forged heat-treated product is 80% or more and the cracking property is further improved, the yield strength and fatigue strength of the forged heat-treated product can be improved and the cracking property can also be improved.

Therefore, the present inventors have found that sufficient martensite and tempered bainite can be obtained even when the total area ratio of tempered martensite and tempered bainite is 80% or more in the microstructure of a steel material (forged heat-treated product) after hot forging and heat treatmentThe cracking steel materials were separated and further investigated and investigated. As a result, they found that: among various oxide-based inclusions, with SiO ₂ The inclusions mainly containing CaO are made of Al in comparison with the inclusions mainly containing CaO ₂ O ₃ Al as a main component ₂ O ₃ The inclusions have an influence on the cracking properties of a forged heat-treated product having a structure in which the sum of the area fractions of tempered martensite and tempered bainite is 80% or more. This point will be described in detail below.

Al is added as a deoxidizer in the deoxidation treatment in the refining step, and is bonded to oxygen in molten steel to form Al ₂ O ₃ . Usually, al ₂ O ₃ Aggregation, combination and floating up occur in the molten steel and are removed. On the other hand, a part of Al ₂ O ₃ Will remain in the steel to form Al ₂ O ₃ Is an inclusion. Here, in the present specification, al ₂ O ₃ The term "inclusions" means Al in the inclusions ₂ O ₃ The inclusion content of (b) exceeds 70.0% by mass. Al remaining in the steel ₂ O ₃ The inclusions are not dissolved in the steel material or the forged heat-treated product and remain as solid solution.

Al in steel ₂ O ₃ The toughness of the inclusions is extremely low as compared with that of the base metal (matrix of the steel). Thus, upon cracking, al ₂ O ₃ Brittle fracture of the inclusions occurs. Brittle-broken Al ₂ O ₃ The inclusions further become the starting point of the destruction in Al ₂ O ₃ The interface between the inclusions and the matrix generates sharp initial cracks. The steel material is easily brittle-broken due to the strong plastic restraint at the tip of the initial crack. Cracks formed by increased brittleness of initial cracks and adjacent Al ₂ O ₃ Cracks generated by the inclusions are combined with each other, thereby obtaining a brittle fracture surface.

According to the above mechanism, even if a steel material (forged heat-treated product) having a microstructure in which the sum of the area ratios of tempered martensite and tempered bainite having high toughness is 80% or more is formed after hot forging and heat treatment, al is included in the steel material ₂ O ₃ The initial cracks are generated by inclusion, and the brittle cracks are easily increased. Therefore, the fracture surface becomes a brittle fracture surface, and the ductile fracture surface is suppressed. As a result, the steel material after hot forging and heat treatment can have excellent cracking properties.

On the other hand, as deoxidizers other than Al, si, ca, and the like are also widely used. Si and Ca form SiO in molten steel ₂ And CaO. SiO 2 ₂ The fatigue strength and hot workability of the steel material tend to be lowered. In addition, caO and Al ₂ O ₃ Has higher toughness than Al, therefore ₂ O ₃ It is more difficult to improve the cracking properties of the steel material after hot forging and heat treatment.

As described above, in order to improve the cracking property of the steel material after hot forging and heat treatment while maintaining the hot workability of the steel material, it is appropriate to use Al among the oxide-based inclusions in the steel ₂ O ₃ Containing inclusions, not using SiO ₂ And CaO. Based on the above idea, the present inventors further focused on Al ₂ O ₃ The proper number density of the inclusions was investigated and investigated. As a result, they found that: if Al is 3 μm or more in √ AREA ₂ O ₃ Inclusions (hereinafter, also referred to as "coarse Al ₂ O ₃ Inclusions) of 0.05 to 1.00 pieces/mm ² As a result of the object of improving the yield strength and fatigue strength of the forged heat-treated product while maintaining the hot workability of the steel material, even when the microstructure of the steel material after hot forging and heat treatment (forged heat-treated product) assumes a microstructure in which the sum of the area fractions of tempered martensite and tempered bainite is 80% or more, excellent cracking properties can be obtained.

The gist of the steel material of the present application completed based on the above findings is as follows.

[1] The steel material has the following chemical composition:

in mass percent

C：0.10～0.60％、

Si：0.05～1.00％、

Mn：0.30～1.50％、

P: less than 0.1000 percent,

S: less than 0.3000 percent,

Al：0.003～0.100％、

N: less than 0.0200%,

Cr：0～2.50％、

Cu：0～0.60％、

Ni：0～0.60％、

Mo：0～0.70％、

V：0～0.049％、

Ti：0～0.250％、

B：0～0.0050％、

Nb：0～0.100％、

Te：0～0.3000％、

Ca：0～0.0100％、

Bi:0 to 0.4000%, and

and the balance: fe and impurities in the iron-based alloy, and the impurities,

satisfying the formula (1) when the Cr content is 0-0.50%,

when the Cr content is 0.51 to 2.50%, the formula (2) is satisfied.

In the present specification, the "steel material" is not particularly limited. The "steel material" may be a steel material for hot forging, i.e., a steel material for hot forging. In the present specification, "Al ₂ O ₃ The term "inclusions" means Al in the inclusions ₂ O ₃ The inclusion content of (b) exceeds 70.0% by mass.

[2] The steel product according to [1], wherein,

the chemical composition comprises a chemical composition selected from the group consisting of

Cr：0.01～2.50％、

Cu：0.01～0.60％、

Ni：0.01～0.60％、

Mo：0.01～0.70％、

V：0.005～0.049％、

Ti：0.005～0.250％、

B:0.0005 to 0.0050%, and

Nb：0.005～0.100％

1 or 2 or more of the group.

[3] The steel product of (3) is the steel product of [1] or [2], wherein,

Te：0.0003～0.3000％、

Ca:0.0003 to 0.0100%, and

Bi：0.0003～0.4000％、

1 or 2 or more of the group.

[4] The forged heat-treated product of (1) has the following chemical composition:

in mass percent

C：0.10～0.60％、

Si：0.05～1.00％、

Mn：0.30～1.50％、

P: less than 0.1000%,

S: less than 0.3000%,

Al：0.003～0.100％、

N: less than 0.0200%,

Cr：0～2.50％、

Cu：0～0.60％、

Ni：0～0.60％、

Mo：0～0.70％、

V：0～0.049％、

Ti：0～0.250％、

B：0～0.0050％、

Nb：0～0.100％、

Te：0～0.3000％、

Ca：0～0.0100％、

Bi:0 to 0.4000%, and

the balance is as follows: fe and impurities in the iron-based alloy, and the impurities,

the Cr content is 0-0.50%, the formula (1) is satisfied,

satisfying the formula (2) when the Cr content is 0.51-2.50%;

In the microstructure of the forged heat-treated product, the sum of tempered martensite and tempered bainite is 80 area% or more.

[5] The forging heat-treated product of [4], wherein,

Cr：0.01～2.50％、

Cu：0.01～0.60％、

Ni：0.01～0.60％、

Mo：0.01～0.70％、

V：0.005～0.049％、

Ti：0.005～0.250％、

B:0.0005 to 0.0050%, and

Nb：0.005～0.100％

1 or 2 or more of the group.

[6] The forging heat-treated product of [4] or [5], wherein,

Te：0.0003～0.3000％、

Ca:0.0003 to 0.0100%, and

Bi：0.0003～0.4000％、

1 or 2 or more of the group.

[7] The method for producing a forged heat-treated product of (1) comprises the steps of:

a hot forging step of heating the steel material according to any one of [1] to [3] to 1100 to 1300 ℃ and performing hot forging to produce an intermediate product;

The steel material of the present embodiment will be described in detail below. The "%" relating to an element means mass% unless otherwise specified.

[ chemical composition ]

The chemical composition of the steel material of the present invention contains the following elements.

C：0.10～0.60％

Carbon (C) improves the yield strength and fatigue strength of the steel after hot forging and heat treatment. If the C content is too low, the effect cannot be obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the C content is too high, the machinability of the steel material after hot forging and heat treatment is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the C content is 0.10 to 0.60%. The lower limit of the C content is preferably 0.13%, more preferably 0.14%, and still more preferably 0.15%. The upper limit of the C content is preferably 0.55%, more preferably 0.52%, and still more preferably 0.50%.

Si：0.05～1.00％

Silicon (Si) is solid-dissolved into the steel material, and improves the yield strength and fatigue strength of the steel material after hot forging and heat treatment. If the Si content is too low, the effect cannot be obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Si content is too high, the above effects are saturated. If the Si content is too high, the hot workability of the steel material is lowered and the manufacturing cost of the steel material is increased even if the content of other elements is within the range of the present embodiment. Therefore, the Si content is 0.05 to 1.00%. The lower limit of the Si content is preferably 0.06%, more preferably 0.07%, and still more preferably 0.08%. The upper limit of the Si content is preferably 0.99%, more preferably 0.95%, and still more preferably 0.90%.

Mn：0.30～1.50％

Manganese (Mn) deoxidizes a steel material at a molten steel stage in a manufacturing process. Mn further improves the yield strength and fatigue strength of the steel after hot forging and heat treatment. If the Mn content is too low, these effects cannot be obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mn content is too high, the hot workability of the steel material is lowered even if the contents of other elements are within the ranges of the present embodiment. Therefore, the Mn content is 0.30 to 1.50%. The lower limit of the Mn content is preferably 0.33%, more preferably 0.34%, and still more preferably 0.35%. The upper limit of the Mn content is preferably 1.30%, more preferably 1.20%, and still more preferably 1.00%.

P: less than 0.1000%

Phosphorus (P) is an impurity that is inevitably contained. In other words, the P content exceeds 0%. If the P content exceeds 0.100%, the hot workability of the steel material is lowered even if the contents of other elements are within the ranges of the present embodiment. Therefore, the P content is 0.1000% or less, more specifically, the P content exceeds 0% and is 0.1000% or less. The upper limit of the P content is preferably 0.0800%, more preferably 0.0500%. The P content is preferably as low as possible. However, if the P content is reduced to the limit by the refining step, the productivity is lowered and the production cost is increased. Therefore, in consideration of usual operations, the preferable lower limit of the P content is 0.0001%, and more preferably 0.0005%.

S: less than 0.3000%

Sulfur (S) is an impurity inevitably contained. In other words, the S content exceeds 0%. If the S content exceeds 0.300%, the hot workability of the steel material is lowered even if the contents of other elements are within the ranges of the present embodiment. Therefore, the S content is 0.3000% or less, more specifically, the S content exceeds 0% and is 0.3000% or less. The preferable upper limit of the S content is 0.2000%, more preferably 0.1500%. The S content is preferably as low as possible. However, if the S content is reduced to the limit by the refining step, the productivity is lowered and the production cost is increased. Therefore, in consideration of usual operations, the preferable lower limit of the S content is 0.0001%, and more preferably 0.0005%.

Al：0.003～0.100％

Aluminum (Al) deoxidizes steel at the molten steel stage in the manufacturing process. Al bonds with oxygen to form coarse Al ₂ O ₃ Is an inclusion. Coarse Al ₂ O ₃ The inclusions remain in the steel material, and improve the cracking properties of the steel material after hot forging and heat treatment. If the Al content is too low, these effects cannot be obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the content of Al is too high, coarse Al is excessively generated even if the content of other elements is within the range of the present embodiment ₂ O ₃ Inclusions decrease the fatigue strength of the steel after hot forging and heat treatment. In this case, the hot workability of the steel material is further lowered. If the Al content is too high, the production cost further increases. Therefore, the Al content is 0.003 to 0.100%. The lower limit of the Al content is excellentIt is preferably 0.004%, more preferably 0.005%, still more preferably 0.006%, still more preferably 0.011%. The upper limit of the Al content is preferably 0.080%, more preferably 0.060%, and still more preferably 0.050%. In the steel material according to the embodiment of the present invention, the Al content means the total Al content.

N:0.0200% or less

Nitrogen (N) is inevitably contained. In other words, the N content exceeds 0%. N bonds with Al to form AlN, which blocks Al ₂ O ₃ Forming of (3). As a result, the steel material has reduced cracking properties after hot forging and heat treatment. Therefore, the N content is 0.0200% or less, more specifically, the N content exceeds 0% and is 0.0200% or less. The upper limit of the N content is preferably 0.0150%, more preferably 0.0100%. The N content is preferably as low as possible. However, if the N content is reduced to the limit by the refining step, the productivity is lowered and the production cost is increased. Therefore, in consideration of usual operations, the preferable lower limit of the N content is 0.0001%, and more preferably 0.0005%.

The balance of the chemical composition of the steel material of the present embodiment is Fe and impurities. Here, the impurities are elements that are mixed from ores and scraps as raw materials or a production environment in the industrial production of steel materials, and are acceptable within a range that does not adversely affect the steel material of the present embodiment. In the present embodiment, the Pb content in the impurities is further limited as follows.

Pb: less than 0.09%

Lead (Pb) is an impurity. Pb may not be contained. That is, the Pb content may be 0%. On the other hand, if the Pb content exceeds 0.09%, the hot workability of the steel material is lowered even if the contents of other elements are within the ranges of the present embodiment. That is, if the content of Pb in the steel material of the present embodiment is 0.09% or less, the presence of Pb is acceptable. Therefore, the steel material according to the present embodiment may contain Pb as an impurity in an amount of 0.09% or less.

The impurities include all elements other than the above-mentioned impurities. The number of impurities may be only 1, or 2 or more. Other impurities than the above impurities are, for example, sb, sn, W, co, as, H, mg and the like. These elements may be contained in the following amounts as impurities, for example.

Sb:0.30% or less, sn:0.30% or less, W:0.30% or less, co:0.30% or less, as:0.30% or less, H:0.005% or less and Mg: less than 0.30 percent.

[ against optional elements ]

The steel material of the present embodiment may further contain 1 or 2 or more selected from the group consisting of Cr, cu, ni, mo, V, ti, B, and Nb in place of a part of Fe. These elements all improve the strength of the steel after hot forging and heat treatment.

Cr：0～2.50％

Chromium (Cr) is an optional element, and may or may not be contained. That is, the Cr content may be 0%. When included, cr improves the yield strength and fatigue strength of the steel after hot forging and heat treatment. The above-mentioned effects can be obtained to some extent by containing Cr in a small amount. On the other hand, if the Cr content is too high, the steel material becomes too hard even if the content of other elements is within the range of the present embodiment, and the machinability of the steel material after hot forging and heat treatment is degraded. If the Cr content is too high, the production cost further increases. Therefore, the Cr content is 0 to 2.50%. The lower limit of the Cr content for more effectively obtaining the above-described effect is preferably 0.01%, more preferably 0.03%, further preferably 0.05%, further preferably 0.07%, further preferably 0.09%, further preferably 0.10%. The lower limit of the Cr content for further improving the yield strength and fatigue strength of the steel material after hot forging and heat treatment is preferably 0.51%, more preferably 0.55%, and still more preferably 0.57%. The upper limit of the Cr content is preferably 2.45%, more preferably 2.42%, and still more preferably 2.40%.

Cu：0～0.60％

Copper (Cu) is an optional element, and may be absent. In other words, the Cu content may be 0%. When contained, cu is dissolved into the steel material, and improves the yield strength and fatigue strength of the steel material after hot forging and heat treatment. The above-described effects can be obtained to some extent by containing a small amount of Cu. On the other hand, if the Cu content is too high, not only the manufacturing cost of the steel material becomes high, but also the machinability of the steel material after hot forging and heat treatment is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Cu content is 0 to 0.60%. The lower limit of the Cu content for more effectively improving the above effect is preferably 0.01%, more preferably 0.05%, and still more preferably 0.10%. The upper limit of the Cu content is preferably 0.59%, more preferably 0.55%, and still more preferably 0.50%.

Ni：0～0.60％

Nickel (Ni) is an optional element, and may be absent. In other words, the Ni content may be 0%. When contained, ni is solid-dissolved into the steel material, and improves the yield strength and fatigue strength of the steel material after hot forging and heat treatment. The above-described effects can be obtained to some extent by containing a small amount of Ni. However, if the Ni content is too high, the manufacturing cost becomes high. If the Ni content is too high, the toughness of the steel after hot forging and heat treatment becomes too high even if the contents of other elements are within the ranges of the present embodiment. As a result, a ductile fracture surface is formed on the fracture surface after fracture separation, and the hot forging and heat treatment of the steel material are degraded in cracking property. Therefore, the Ni content is 0 to 0.60%. The lower limit of the Ni content for more effectively improving the above effect is preferably 0.01%, more preferably 0.02%, and still more preferably 0.05%. The upper limit of the Ni content is preferably 0.59%, more preferably 0.58%, and still more preferably 0.55%.

Mo：0～0.70％

Molybdenum (Mo) is an optional element, and may or may not be contained. In other words, the Mo content may be 0%. When contained, mo forms carbides in the steel, and improves the yield strength and fatigue strength of the steel after hot forging and heat treatment. The above-described effects can be obtained to some extent by containing Mo in a small amount. However, if the Mo content is too high, the hardness of the steel material becomes too high even if the content of other elements is within the range of the present embodiment, and the machinability of the steel material after hot forging and heat treatment is lowered. If the Mo content is too high, the production cost further increases. Therefore, the Mo content is 0 to 0.70%. The lower limit of the Mo content for more effectively improving the above effect is preferably 0.01%, more preferably 0.02%, and still more preferably 0.05%. The upper limit of the Mo content is preferably 0.69%, more preferably 0.68%, and still more preferably 0.65%.

V：0～0.049％

Vanadium (V) is an optional element, and may or may not be contained. In other words, the V content may be 0%. When included, V forms carbides in the steel, improving the yield strength and fatigue strength of the steel after hot forging and heat treatment. The above-described effects can be obtained to some extent by containing V in a small amount. However, if the V content is too high, the production cost of the steel material becomes high. If the content of V is too high, the machinability of the steel material after hot forging and heat treatment is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the V content is 0 to 0.049%. The lower limit of the V content for more effectively improving the above effect is preferably 0.005%, more preferably 0.008%, and further preferably 0.010%. The upper limit of the V content is preferably 0.045%, more preferably 0.044%, still more preferably 0.042%, and yet more preferably 0.040%.

Ti：0～0.250％

Titanium (Ti) is an optional element, and may or may not be contained. In other words, the Ti content may be 0%. When contained, ti precipitates as carbide together with V during cooling and heating after hot forging, and improves the yield strength and fatigue strength of the steel after hot forging and heat treatment. The above-mentioned effects can be obtained to some extent by containing Ti in a small amount. However, if the Ti content is too high, the hot workability of the steel material is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Ti content is 0 to 0.250%. The lower limit of the Ti content for more effectively improving the above effect is preferably 0.005%, more preferably 0.008%, and further preferably 0.010%. The upper limit of the Ti content is preferably 0.240%, more preferably 0.220%, and still more preferably 0.200%.

B：0～0.0050％

Boron (B) is an optional element and may be absent. In other words, the B content may be 0%. When B is contained, B is dissolved in the steel material to improve the hardenability of the steel material. As a result, the yield strength and fatigue strength of the steel material after hot forging and heat treatment are improved. If B is contained to some extent, the above-mentioned effects can be obtained to some extent. However, if the content of B is too high, even if the content of other elements is within the range of the present embodiment, coarse nitrides precipitate in the steel material, and the hot workability of the steel material is reduced. Therefore, the B content is 0 to 0.0050%. The lower limit of the B content for more effectively enhancing the above effect is preferably 0.0005%, more preferably 0.0008%, and still more preferably 0.0010%. The upper limit of the B content is preferably 0.0045%, more preferably 0.0042%, and still more preferably 0.0040%.

Nb：0～0.100％

Niobium (Nb) is an optional element, and may not be contained. In other words, the Nb content may be 0%. When contained, nb forms carbides in the steel material, improving the yield strength and fatigue strength of the steel material after hot forging and heat treatment. The above-described effects can be obtained to some extent by containing a small amount of Nb. However, if the Nb content is too high, the hardness of the steel material becomes too high even if the content of other elements is within the range of the present embodiment, and the machinability of the steel material after hot forging and heat treatment is degraded. Therefore, the Nb content is 0 to 0.100%. The lower limit of the Nb content for more effectively improving the above effect is preferably 0.005%, more preferably 0.010%, and still more preferably 0.015%. The upper limit of the Nb content is preferably 0.095%, more preferably 0.090%, and still more preferably 0.085%.

The steel material of the present invention may further contain 1 or 2 or more selected from the group consisting of Te, ca and Bi in place of a part of Fe. These elements all improve the machinability of the steel material after hot forging and heat treatment.

Te：0～0.3000％

Tellurium (Te) is an optional element, and may or may not be contained. In other words, the Te content may be 0%. When contained, te improves the machinability of the steel material after hot forging and heat treatment. The above-described effects can be obtained to some extent if Te is contained in a small amount. However, if the Te content is too high, the hot workability of the steel material is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Te content is 0 to 0.3000%. The lower limit of the Te content for more effectively enhancing the above effect is preferably 0.0003%, more preferably 0.0005%, and still more preferably 0.0010%. The upper limit of the Te content is preferably 0.2900%, more preferably 0.2500%, and still more preferably 0.2000%.

Ca：0～0.0100％

Calcium (Ca) is an optional element, and may or may not be contained. In other words, the Ca content may be 0%. When included, ca improves the machinability of the steel material after hot forging and heat treatment. The above-mentioned effect can be obtained to some extent by containing a small amount of Ca. However, if the Ca content is too high, the hot workability of the steel material is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Ca content is 0 to 0.0100%. The lower limit of the Ca content for more effectively enhancing the above effect is preferably 0.0003%, more preferably 0.0005%, and still more preferably 0.0010%. The upper limit of the Ca content is preferably 0.0090%, more preferably 0.0080%, and still more preferably 0.0050%.

Bi：0～0.4000％

Bismuth (Bi) is an optional element, and may or may not be contained. In other words, the Bi content may be 0%. When contained, bi improves the machinability of the steel material after hot forging and heat treatment. The above-mentioned effects can be obtained to some extent by containing Bi in a small amount. However, if the Bi content is too high, the hot workability of the steel material is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Bi content is 0 to 0.4000%. The lower limit of the Bi content for more effectively enhancing the above effect is preferably 0.0003%, more preferably 0.0005%, and still more preferably 0.0010%. The upper limit of the Bi content is preferably 0.3900%, more preferably 0.3000%, and still more preferably 0.2000%.

[ concerning the formulas (1) and (2) ]

In the chemical composition of the steel material of the present embodiment, the formula (1) is satisfied when the Cr content is 0 to 0.50%, and the formula (2) is satisfied when the Cr content is 0.51 to 2.50%.

Here, the content (mass%) of the corresponding element is substituted at the symbol of the element in formula (1) and formula (2). In addition, when no corresponding element is contained, the symbol of the element is substituted by "0". fB in the formulae (1) and (2) is "0" when the B content (mass%) is 0% and "1" when the B content (mass%) exceeds 0%.

fn1 (= 7.6 √ C × (1 +0.6 Si) × (1 +4 Mn) × (1-0.6S) × (1 + 0.3Cu) × (1 + 0.5Ni) × (1 + 2Cr) × (1 + 3Mo) × (1 + (1.5 × (0.9-C) × fB))) is an index of the hardenability and segregation of the steel having the above chemical composition.

[ the range of fn1 when the Cr content is 0 to 0.50% ]

When the Cr content is 0 to 0.50%, if fn1 is less than 9, the hardenability of the steel material cannot be sufficiently obtained. In this case, in the microstructure of the steel material after hot forging and heat treatment (forged heat-treated product), the sum of the area fractions of tempered martensite and tempered bainite does not reach 80% or more, and the yield strength and/or fatigue strength of the steel material after hot forging and heat treatment is lowered. On the other hand, if fn1 exceeds 130, segregation of the alloying elements occurs in the steel material. At this time, the fatigue strength of the steel material after hot forging and heat treatment is reduced.

Therefore, when the Cr content is 0 to 0.50%, fn1 is 9 to 130. The lower limit of fn1 is preferably 11, more preferably 13, and further preferably 15. The upper limit of fn1 is preferably 125, more preferably 120, and still more preferably 115.

[ the range of fn1 when the Cr content is 0.51-2.50% ]

When the Cr content is 0.51 to 2.50%, if fn1 is less than 40, the hardenability of the steel material cannot be sufficiently obtained. In this case, in the microstructure of the steel material after hot forging and heat treatment (forged heat-treated product), the sum of the area ratios of tempered martensite and tempered bainite is less than 80%, and the yield strength and/or fatigue strength of the steel material after hot forging and heat treatment is lowered. On the other hand, if fn1 exceeds 300, segregation of the alloying elements occurs in the steel material. At this time, the fatigue strength of the steel material after hot forging and heat treatment is reduced.

Therefore, when the Cr content is 0.51 to 2.50%, fn1 is 40 to 300. The lower limit of fn1 is preferably 42, more preferably 44, and still more preferably 46. The upper limit of fn1 is preferably 295, more preferably 290, and still more preferably 285.

[ coarse Al ₂ O ₃ Number density of inclusions]

In the steel material of the present embodiment, V.AREA is 3 μm or more of Al ₂ O ₃ Inclusions (i.e., coarse Al) ₂ O ₃ Inclusions) of 0.05 to 1.00 pieces/mm ² . As mentioned above, al ₂ O ₃ The term "inclusions" means that the inclusions contain more than 70.0% by mass of Al ₂ O ₃ The inclusion of (2).

Coarse Al if ₂ O ₃ The number density of the inclusions is less than 0.05 pieces/mm ² The steel material after hot forging and heat treatment cannot be sufficiently cracked. On the other hand, if coarse Al is contained ₂ O ₃ The number density of the inclusions exceeds 1.00/mm ² Although excellent cracking properties can be obtained, the fatigue strength and hot workability of the steel material after hot forging and heat treatment are reduced. If coarse Al is contained ₂ O ₃ The number density of the inclusions is 0.05 to 1.00 pieces/mm ² Even if the total area ratio of tempered martensite and tempered bainite in the microstructure of the steel material is 80% or more by the hot forging and the heat treatment, the hot workability of the steel material and the fatigue strength of the steel material after the hot forging and the heat treatment can be maintained, and excellent cracking property can be obtained in the steel material after the hot forging and the heat treatment.

Coarse Al for further improving the cracking properties of steel after hot forging and heat treatment ₂ O ₃ The lower limit of the number density of the inclusions is preferably 0.07/mm ² More preferably 0.10 pieces/mm ² More preferably 0.11 pieces/mm ² More preferably 0.12/mm ² . Coarse Al for further improving hot workability of steel material and fatigue strength of steel material after hot forging and heat treatment ₂ O ₃ The upper limit of the number density of the inclusions is excellentIs selected to be 0.80/mm ² More preferably 0.60 pieces/mm ² 。

Coarse Al ₂ O ₃ The number density of the inclusions can be measured by the following method. When the steel material is a steel bar, a sample is taken from the R/2 portion in the cross section perpendicular to the axial direction (rolling direction) of the steel bar. The R/2 part refers to: in a cross section perpendicular to the axial direction of the bar steel, the center of a line segment (radius R) connecting the center and the surface is located at the center. Out of the surfaces of the samples, 30 samples each having a length of 4mm × a width of 2.5mm were sampled from a surface corresponding to a cross section (vertical cross section) including the axial direction of the bar. The observation surfaces of 30 samples were observed by an optical microscope at 200 magnifications without etching, and a photographic image was generated. The sum of the inspected areas is 300mm ² 。

The inclusions in the observation surface (4 mm. Times.2.5 mm) of each sample were determined from the contrast. Oxide-based inclusions are determined from the determined inclusions based on the shape and contrast of the inclusions. The content (mass%) of the element in each oxide inclusion was measured by an electron beam microanalyzer (EPMA) with respect to the oxide inclusion thus identified. Calculating Al in oxide inclusions from the analyzed contents of the respective elements ₂ O ₃ In the above-mentioned amount. Specifically, any 3 points of the inclusions were determined, and the Al content (% by mass) was measured by using an electron beam having a beam diameter of 1 μm. Using Al and Al ₂ O ₃ Based on the determined Al content, al is calculated ₂ O ₃ Content (% by mass). Obtaining the calculated Al ₂ O ₃ Average content, defined as Al ₂ O ₃ In mass%. Instead of specifying oxide-based inclusions based on shape and contrast, the EPMA-based elemental analysis may be performed on all inclusions in the observation surface, and when any one or more of Al, ca, si, and Mg is contained and oxygen (O) is contained, the inclusions may be specified as oxide-based inclusions.

In the range of the chemical composition of the steel material according to the present embodiment, most of the oxides contained in the oxide inclusions are Al ₂ O ₃ 、CaO、SiO ₂ And MgO, the other oxides being negligible. Therefore, in the present embodiment, al in the inclusions is defined as follows ₂ O ₃ Content (mass%).

Among the oxide inclusions, 3 points are arbitrarily defined. For the determined points, the contents (% by mass) of Al, ca, si and Mg were measured using an electron beam having a beam diameter of 1 μm. The measured contents of the respective elements are converted into the contents of the corresponding oxides, and defined as calculated values of the respective oxides. More specifically, al is multiplied by the Al content (% by mass) measured by EPMA ₂ O ₃ Atomic weight ratio to Al (= Al) ₂ O ₃ Molecular weight of (Al)/(atomic weight of Al × 2)), and obtaining Al at the specified point ₂ O ₃ Calculated value of (mass%).

For CaO and SiO ₂ And MgO, also with Al ₂ O ₃ CaO and SiO were determined in the same manner ₂ And calculated value (mass%) of MgO. Determination of Al ₂ O ₃ The ratio of the calculated value of (a) to the sum of the calculated values of the oxides determined is defined as Al at an arbitrary specified point ₂ O ₃ Content (mass%). Al of the determined 3 points ₂ O ₃ The arithmetic mean of the contents (% by mass) is defined as "Al in inclusions ₂ O ₃ Content (% by mass) ".

Al in inclusions to be identified by the above method ₂ O ₃ The inclusion content (mass%) exceeding 70.0% is determined as Al ₂ O ₃ Is an inclusion. Calculating each of the determined Al by using an image analysis device ₂ O ₃ V-AREA which is an inclusion. Specifically, each of the determined Al is obtained ₂ O ₃ The length L (. Mu.m) and width W (. Mu.m) of the inclusion. Suppose each Al ₂ O ₃ The inclusions are rectangular and have an area (= L × W (μm) ² ) Is obtained in the form of (c). The square root of the area obtained was determined and defined as Al ₂ O ₃ V AREA (μm) of inclusion series.

Each Al was obtained ₂ O ₃ After v AREA of inclusions, coarse Al of 3 μm or more is determined ₂ O ₃ Is an inclusion. Finding the determined coarse Al ₂ O ₃ The number of inclusions was divided by the total area of the specimens (300 mm) ² ) The value thus obtained is defined as coarse Al ₂ O ₃ Number density of inclusions/mm ² )。

[ production method ]

An example of the method for producing the steel material will be described. The manufacturing method of this example includes a refining step, a casting step, and a hot working step. Hereinafter, a method for producing a steel bar will be described specifically as an example of the steel material.

[ refining step ]

Molten steel satisfying the above chemical composition and formula (1) (the Cr content is 0 to 0.50%) or formula (2) (the Cr content is 0.51 to 2.50%) is produced by a known method. Specifically, decarburization, dephosphorization and desiliconization in the converter are performed by a known method. After tapping, an aluminum deoxidizer is added to the ladle to perform deoxidation treatment. Note that, in order to prevent SiO ₂ And mixing CaO, wherein a ladle is an aluminum deoxidation special ladle. In addition, as the aluminum deoxidizer, a metal Al or an Al alloy having an Al content of 80% by mass or more is used.

After the deoxidation treatment, vacuum degassing treatment was performed. Here, the Al content in the molten steel is adjusted by confirming the molten steel components in the production process and adding the aluminum deoxidizer (metallic Al or Al alloy having an Al content of 80% by mass or more) to the vacuum degassing treatment. The aluminum deoxidizer added in the vacuum degassing treatment accounts for 50 to 70% by mass of the entire aluminum deoxidizer to be added.

Note that, in order to suppress SiO ₂ The addition of Si is carried out after the steel is sufficiently deoxidized by an aluminum deoxidizer. The addition of Si is performed after 10 minutes or more has elapsed since the addition of the additional aluminum deoxidizer, for example. Furthermore, in order to make Al ₂ O ₃ The molten steel is accumulated in an appropriate range, and a preferable holding time for the molten steel temperature to be 1600 ℃ or more from the time when the deoxidizer is added after tapping to the time when casting is started is 15 to 60 minutes. The lower limit of the preferable time for the molten steel temperature to be 1600 ℃ or higher is 30 minutes, and more preferably 40 minutes. By refining as described aboveA step of obtaining Al satisfying the above chemical composition, formula (1) and inclusion regulation and having √ AREA as 3 μm or more ₂ O ₃ Inclusions (i.e., coarse Al) ₂ O ₃ Inclusions) of 0.05 to 1.00 pieces/mm ² The molten steel of (1).

[ casting Process ]

Using the molten steel, a cast slab (slab or bloom) or a steel ingot (ingot) is produced by a known method. Examples of the casting method include a continuous casting method and an ingot casting method.

[ Hot working Process ]

In the hot working step, the cast slab or steel ingot produced in the casting step is hot worked to produce a steel material. The steel material is, for example, bar steel. The hot working step is performed by a known method. The hot working process includes, for example, a rough rolling process and a finish rolling process. The rough rolling step is, for example, rough rolling using a blooming mill. The finish rolling step is, for example, rolling using a continuous rolling mill. In a continuous rolling mill, a horizontal stand having a pair of horizontal rolls and a vertical stand having a pair of vertical rolls are alternately arranged in a row. The heating temperature in the rough rolling step is, for example, 1000 to 1300 ℃. The heating temperature in the finish rolling step is, for example, 1000 to 1300 ℃. In a heating temperature region of 1000 to 1300 ℃, al ₂ O ₃ The form of the inclusions is not particularly changed. The hot working step may be performed by hot forging instead of hot rolling. In the above description, the hot working step includes two steps, i.e., the rough rolling step and the finish rolling step, but only the finish rolling step may be performed without the rough rolling step.

The steel material is produced by the above-described production process. In the above-described manufacturing method, the steel bar is manufactured as a steel material, but the steel material according to the embodiment of the present invention may be a wire rod. The cross section perpendicular to the axial direction of the steel material is not particularly limited. The cross-sectional shape perpendicular to the axial direction of the steel material is, for example, rectangular, circular, oval, or polygonal.

The method for producing the steel material according to the present embodiment is not limited to the above-described production method. The above-described manufacturing method is one of the preferred manufacturing methods, but by other manufacturing methodsThe steel material of the present embodiment can also be produced by the method. Provided that √ AREA in the steel is 3 μm or more Al ₂ O ₃ The number density of the inclusions reaches 0.05 to 1.00 pieces/mm ² The above-mentioned production method is not particularly limited.

[ method for producing forged Heat-treated product ]

An example of a method for producing a forged heat-treated product using the steel material will be described. The manufacturing method of this example includes a hot forging step, a quenching step, and a tempering step. Specifically, a method for manufacturing a fracture splitting connecting rod will be described as an example of a forged heat-treated product.

[ Hot forging Process ]

In the hot forging step, the steel material is heated to 1100 to 1300 ℃ and hot forged to produce an intermediate product. Specifically, the steel material is heated by a high-frequency induction heating furnace. In the hot forging step of the present embodiment, the heating temperature is preferably 1100 to 1300 ℃, and the heating time is preferably 10 to 15 minutes. Since the heating temperature in the high-frequency induction heating furnace is low, al in the steel material is low ₂ O ₃ The form of the inclusions is not particularly changed.

In the hot forging step of the present embodiment, the lower limit of the heating temperature is more preferably 1120 ℃, still more preferably 1140 ℃, still more preferably 1160 ℃. The upper limit of the heating temperature is more preferably 1280 ℃, still more preferably 1260 ℃, still more preferably 1240 ℃.

The heated steel material is hot forged to produce an intermediate product (e.g., a roughly shaped split connecting rod). The degree of working at the time of hot forging is not particularly limited. The degree of working at the time of hot forging is preferably 0.22 or more. Here, the degree of working is set to the maximum value of the logarithmic strain generated in the portion other than the burr in the hot forging step.

[ quenching Process ]

In the quenching step, quenching is performed to cool the intermediate product after the hot forging step, with an average cooling rate of between 800 ℃ and 100 ℃ being 10 to 200 ℃/sec. In the present specification, "quenching" means that A is ₃ Cooling the steel above the point. Preferably at least 800 to 100 ℃ after hot forgingThe intermediate product of (4) is continuously cooled. In this case, the average cooling rate at 800 to 100 ℃ is preferably 10 to 200 ℃/sec.

If the cooling rate is too slow, a microstructure in which the sum of the area fractions of tempered martensite and tempered bainite is 80% or more may not be obtained, and the yield strength and fatigue strength of the forged heat-treated product may not be sufficiently obtained. On the other hand, if the cooling rate is too high, the temperature difference in the cross section of the forged product may increase. At this time, a time difference occurs between the phase transition timings of the surface layer and the interior. As a result, tensile residual stress is generated in the surface layer after quenching, and therefore, the fatigue strength of the forged heat-treated product may not be sufficiently obtained. Therefore, the cooling rate is preferably set to 10 to 200 ℃/sec.

The lower limit of the cooling rate is more preferably 12 ℃/sec, still more preferably 15 ℃/sec, still more preferably 18 ℃/sec, and still more preferably 20 ℃/sec. The upper limit of the cooling rate is more preferably 190 ℃/sec, still more preferably 185 ℃/sec, and still more preferably 180 ℃/sec.

If the cooling start temperature during quenching is too low, a desired microstructure may not be obtained in the steel material after hot forging and heat treatment (forged heat-treated product). Therefore, the cooling start temperature during quenching is preferably 800 ℃ or higher. Further, if the cooling stop temperature during quenching is too high, a desired microstructure may not be obtained in the steel material after hot forging and heat treatment (forged heat-treated product). Therefore, the cooling stop temperature during quenching is preferably 100 ℃ or lower. Therefore, in the quenching according to the present embodiment, it is preferable to cool the intermediate product after the hot forging step at an average cooling rate of 10 to 200 ℃/sec between 800 ℃ and 100 ℃. In this case, the cooling rate is determined by the temperature measured at the portion of the intermediate product that is cooled the slowest in the cross section. For example, when the intermediate product is a bar, it is determined by the temperature measured at the center of a cross section perpendicular to the axial direction of the bar.

The quenching may be performed immediately after the hot forging, or may be performed after the intermediate product is reheated after the hot forging. The cooling method in quenching is not particularly limited as long as it is a continuous cooling method, and examples thereof include water cooling and oil cooling.

[ tempering step ]

In the tempering step, the intermediate product after the quenching step is tempered by holding the intermediate product at 400 to 650 ℃ for 30 to 90 minutes. In the present specification, "tempering" means reheating a steel material to A ₁ Below the point and held. In the tempering treatment of the present embodiment, the tempering temperature is preferably 400 to 650 ℃. When the tempering temperature is too low, tensile residual stress may occur on the surface of the forged heat-treated product, and the fatigue strength may be lowered. On the other hand, if the tempering temperature is too high, tempering softening occurs, and the yield strength and/or fatigue strength of the forged heat-treated product may decrease.

Therefore, the tempering temperature is preferably set to 400 to 650 ℃. The lower limit of the tempering temperature is more preferably 410 ℃, still more preferably 420 ℃, and still more preferably 430 ℃. The upper limit of the tempering temperature is more preferably 640 ℃, still more preferably 630 ℃, still more preferably 620 ℃. In the tempering treatment of the present embodiment, the holding time (tempering time) is preferably 30 to 90 minutes. In the present specification, the tempering temperature refers to the temperature of a furnace used when reheating a steel material. In the present specification, the tempering time is a time during which the temperature of the steel material is maintained in a range of ± 5 ℃ from the temperature of the furnace used in reheating.

The forged heat-treated product of the present embodiment is produced through the above-described steps. The forged heat-treated product after tempering may be subjected to rough cutting by machining as necessary. As an example of the forged heat-treated product, in the case of manufacturing a split connecting rod, the forged heat-treated product subjected to the tempering treatment is subjected to machining. The forged heat-treated product roughly cut by machining is subjected to fracture splitting (cracking) of the large head 100. The forged heat-treated product after fracture splitting is subjected to finish cutting to produce a final fracture splitting connecting rod. The split connecting rod is manufactured through the above-described process.

In the above-described method for producing a forged heat-treated product, the method for producing a fracture splitting connecting rod has been described as an example of the forged heat-treated product, but the forged heat-treated product is not limited to the fracture splitting connecting rod. The forged heat-treated product may be a member for other machine structural use.

[ microstructure of forged Heat-treated product ]

The microstructure of the produced forged heat-treated product is not particularly limited. However, when a steel material having the above chemical composition is subjected to hot forging and heat treatment to produce a forged heat-treated product in order to improve yield strength and fatigue strength, the total of the area ratios of tempered martensite and tempered bainite in the microstructure of the steel material after hot forging and heat treatment (forged heat-treated product) may be 80% or more.

In the microstructure of the forged heat-treated product, when the sum of the area ratios of tempered martensite and tempered bainite is not 100%, the balance of the matrix structure is ferrite or ferrite and pearlite. The lower limit of the sum of the area ratios of tempered martensite and tempered bainite in the microstructure is preferably 85%, more preferably 90%, still more preferably 95% or more, and most preferably 100%. An example of the sum of the area ratios of tempered martensite and tempered bainite is 95 to 100%.

As a result of producing a forged heat-treated product by hot forging and heat treatment of a steel material having the above chemical composition in order to improve yield strength and fatigue strength, it is assumed that the total of the area fractions of tempered martensite and tempered bainite in the microstructure of the forged heat-treated product is 80% or more. Further, a case is assumed where the forged heat-treated product is a split connecting rod. At this time, when the large head 100 is broken and divided into 2 members (the cap 2 and the stem 3), the broken portion is plastically deformed, the broken surface is likely to have a ductile section, and the cracking property is likely to be lowered. However, the steel material of the present embodiment contains Al in an amount exceeding 70.0% by mass ₂ O ₃ Al of (2) ₂ O ₃ Among inclusions, coarse Al having a V AREA of 3 μm or more ₂ O ₃ The number density of the inclusions is 0.05 to 1.00 pieces/mm ² . Therefore, even when the total of the area fractions of tempered martensite and tempered bainite reaches 80% or more in the structure of the forged heat-treated product produced by hot forging and heat treatment of the steel material according to the present embodiment, the fracture surface of the forged heat-treated product easily forms a brittle fracture surface, and excellent cracking properties can be maintained.

As described above, in the steel material according to the present embodiment, even when a forged heat-treated product in which the sum of the area ratios of tempered martensite and tempered bainite is 80% or more is produced, excellent cracking property can be obtained. Therefore, even when the total area ratio of tempered martensite and tempered bainite in the microstructure is less than 80% as a result of hot forging and heat treatment performed on the steel material of the present embodiment, the forged heat-treated product has excellent cracking properties.

The total area ratio of tempered martensite and tempered bainite in the microstructure of the steel material (forged heat-treated product) after hot forging and heat treatment can be measured by the following method. In the forged heat-treated product, 10 samples were taken from a portion (inner region) other than a region (surface layer region) up to a depth position of 1mm or more from the surface. Any surface of each sample collected was used as an observation surface. The observation surface was polished and then etched with 3% nital (nital etching solution). The etched observation surface was observed with an optical microscope of 200 magnifications, and a photographic image of arbitrary 5 fields of view was generated. The area of each field was 475. Mu. M.times.475. Mu.m.

In each field, the contrast of each phase, such as ferrite, pearlite, tempered martensite, and tempered bainite, differs. Therefore, tempered martensite and tempered bainite in each field of view are determined based on the contrast. In the present specification, tempered martensite is not distinguished from tempered bainite. Therefore, in the present specification, regions other than ferrite and pearlite in each field of view are defined as "tempered martensite and tempered bainite". Determining the areas (. Mu.m) of the tempered martensite and tempered bainite ² ). The ratio of the sum of the areas of tempered martensite and tempered bainite in all the fields of view to the total area of all the fields of view (5 fields × 10) is defined as the sum (%) of the area ratios of tempered martensite and tempered bainite.

The steel material of the present embodiment has a high yield strength when it is heated to 1100 to 1300 ℃ and hot forged to produce an intermediate product, the intermediate product is cooled at an average cooling rate of 10 to 200 ℃/sec at 800 to 100 ℃, and then the intermediate product is held at 400 to 650 ℃ for 30 to 90 minutes. The high yield strength specifically means: the yield strength obtained by a tensile test according to JIS Z2241 (2011) is 751MPa or more when the Cr content is 0 to 0.50% or less, and 851MPa or more when the Cr content is 0.51 to 2.50%.

The steel material of the present embodiment is heated to 1100 to 1300 ℃ and hot forged to produce an intermediate product, and after cooling the intermediate product at an average cooling rate of 10 to 200 ℃/sec between 800 ℃ and 100 ℃, the intermediate product is held at 400 to 650 ℃ for 30 to 90 minutes, thereby exhibiting high fatigue strength. The high fatigue strength specifically means: the fatigue strength obtained by an alternating stress fatigue test based on a sine wave of JIS Z2273 (1978) and having a phase of 0 (MPa) is 501MPa or more when the Cr content is 0 to 0.50% or less, and is 551MPa or more when the Cr content is 0.51 to 2.50%.

The steel material of the present embodiment has excellent cracking properties when it is heated to 1100 to 1300 ℃ and hot forged to produce an intermediate product, the intermediate product is cooled at an average cooling rate of 10 to 200 ℃/sec at 800 to 100 ℃, and then the intermediate product is held at 400 to 650 ℃ for 30 to 90 minutes. The excellent cracking property can be evaluated by the following method. A test piece 10 shown in fig. 2A was produced by machining, in which a hole 11 was formed in the center and V-shaped notches M were formed in two locations corresponding to the respective end points of the diameter. After a jig 12 having a hole 14 for driving a wedge 13 formed in the center thereof is inserted into the hole 11, the wedge 13 is driven into the hole 14, and the test piece 10 is separated into two

members

10A and 10B by breaking. The vicinities of both side surfaces of the

members

10A and 10B obtained by the fracture separation are subjected to bolt hole processing, and the

members

10A and 10B are fastened with bolts. The maximum value Dmax and the minimum value Dmin of the diameter of the hole 11 of the test piece 10 after the fracture separation and after the fastening of the bolt 15 were measured, and the difference was defined as the inside diameter deviation difference Δ D (= Dmax-Dmin, in μm). The excellent cracking property means that the difference Δ D in the inside diameter is 40 μm or less.

The steel material of the present embodiment will be described in more detail below with reference to examples.

[ example 1]

In example 1, a steel material having a Cr content of 0 to 0.50% was examined. Specifically, molten steels having chemical compositions shown in table 1 were produced. In table 1, "-" indicates that the content of the corresponding element is at an impurity level.

[ Table 1]

TABLE 1

Referring to Table 1, the chemical compositions of test Nos. E-1 to E-45 and C-6 to C-18 are suitable and satisfy formula (1). On the other hand, the chemical compositions of test Nos. C-1 to C-5 are not suitable or do not satisfy formula (1). The chemical composition of test No. C-5 is within the range of the chemical composition of the steel described in patent document 5.

The molten steel of each test number was subjected to primary refining in a 70-ton converter, and was tapped into a ladle. In test No. other than test No. C-6, the ladle was made to prevent SiO ₂ For mixing CaO, an aluminum deoxidation package (indicated by "a" in the column of "package" in table 2) was used. In test No. C-6, the same packets as those for silicon deoxidation and calcium deoxidation were used instead of the dedicated packet for aluminum deoxidation (indicated by "E" in the column of "dedicated packet" in Table 2).

[ Table 2]

TABLE 2

Immediately after tapping to a ladle, an aluminum deoxidizer is added to perform deoxidation treatment. In the other test numbers except test number C-7, the aluminum deoxidizer used was one having an Al content of 80% by mass or more (in Table 2, the column entitled "Al ratio of deoxidizer" is "A"). On the other hand, in test No. C-7, an aluminum deoxidizer having an Al content of less than 80% was used (in the column of "deoxidizer Al ratio" in Table 2, it is referred to as "E").

In the test Nos. other than the test Nos. C-9 and C-10, an aluminum deoxidizer (deoxidizer having an Al content of 80% by mass or more) was added to the molten steel in the vacuum degassing treatment after the deoxidation treatment.

Here, when the amount of the aluminum deoxidizer added in the vacuum degassing treatment is 50 to 70 mass% of the entire aluminum deoxidizer added in the refining step, it is judged that the deoxidizer addition rate is appropriate (in the column of "additional deoxidizer" in table 2, it is referred to as "a"). On the other hand, when the amount of the aluminum deoxidizer added in the vacuum degassing is less than 50% of the total amount of the aluminum deoxidizer added in the refining step, it is judged that the deoxidizer addition rate in the vacuum degassing treatment does not satisfy the condition (in the column of "additional deoxidizer" in table 2, it is referred to as "LE"). Further, when the amount of the aluminum deoxidizer added in the vacuum degassing treatment exceeds 70% of the total amount of the aluminum deoxidizer added in the refining step, it is judged that the deoxidizer addition rate does not satisfy the condition (in the column of "additional deoxidizer" in table 2, it is referred to as "UE"). In any of the test numbers, after 10 minutes or more from the addition of the aluminum deoxidizer in the vacuum degassing treatment, si was further added.

In the test Nos. other than test Nos. E-38, E-39, C-5, C-11 and C-12, the molten steel temperature was adjusted so that the time for which the molten steel temperature became 1600 ℃ or more became 40 minutes from the addition of the aluminum deoxidizer to the molten steel immediately after tapping until the start of casting (in Table 2, the column of "holding time of 1600 ℃ or more" is referred to as "A"). In test No. E-38, the holding time for a molten steel temperature of 1600 ℃ or higher was 30 minutes (in Table 2, the column entitled "holding time of 1600 ℃ or higher" is referred to as "B"), and in test No. E-39, the holding time for a molten steel temperature of 1600 ℃ or higher was 15 minutes (in Table 2, the column entitled "holding time of 1600 ℃ or higher" is referred to as "C").

In test No. C-11, the molten steel temperature was adjusted so that the time for which the molten steel temperature was 1600 ℃ or more reached 70 minutes (hereinafter referred to as "UE" in the column of "retention time of 1600 ℃ or more" in table 2) from the time when the aluminum deoxidizer was added to the molten steel immediately after tapping until the time when casting was started. In test Nos. C-5 and C-12, the molten steel temperature was adjusted so that the time during which the molten steel temperature was 1600 ℃ or more reached 5 minutes (hereinafter referred to as "LE" in the column of "holding time of 1600 ℃ or more" in Table 2) from the time when the aluminum deoxidizer was added to the molten steel immediately after tapping until the time when casting was started.

In addition, in the other test numbers except for test number C-8, si was further added after 10 minutes or more from the addition of the aluminum deoxidizer at the time of vacuum degassing (in the column of "Si added" in Table 2, it is referred to as "A"). On the other hand, in test No. C-8, si was added (in Table 2, the column of "Si added" is referred to as "E") when the time period was less than 10 minutes after the aluminum deoxidizer was added during the vacuum degassing.

Next, for the molten steel of each test number, a cast slab (bloom) was produced from the molten steel by a continuous casting method using a continuous casting machine. The cross section of the bloom is 300mm x 400mm.

The produced bloom is hot-rolled to produce a strip billet. First, a bloom was heated at 1150 ℃ for 100 minutes, and then blooming was performed using a blooming mill to produce a strip billet. Subsequently, the bar was heated at 1150 ℃ for 35 minutes, and then finish rolled by a finish rolling mill to produce a bar having a diameter of 40 mm. A steel material (steel bar) is produced by the above-described production process.

[ production of forging Heat treatment simulation product ]

The steel material (bar) was cut in a direction perpendicular to the longitudinal direction, and a sample having a diameter of 40mm and a length of 100mm was collected. The test materials of the respective test numbers were heated and held for 5 minutes. In Table 2, the heating temperature of the test materials of each test number during hot forging is shown in the column of "forging heating temperature (. Degree. C.)". After heating, 90% thermal compression was rapidly performed in the axial direction, and the resulting material was molded into a disk shape. Next, the test piece molded into a disk shape is immersed in oil at 50 to 150 ℃ and cooled to 100 ℃ or lower. The temperature at which cooling is started is 800 ℃ or higher. In table 2, the cooling rate of the test material of each test number is shown in the column of "quench cooling rate (c/sec)". The cooling rate is determined by inserting a thermocouple into a hole opened from the side surface to the center portion of the test piece by machining, and measuring the temperature from the center portion of the test piece.

Tempering was performed by reheating and holding the cooled test materials of each test number for 30 minutes. The tempering temperature (. Degree. C.) of the test materials of each test number is shown in the column "tempering temperature (. Degree. C.)". The tempering temperature is set to the temperature (c) of the furnace used for reheating. The tempering time was set to a time (minutes) during which the test piece was held at the tempering temperature ± 5 ℃ described in table 2. The forging heat treatment dummy is manufactured by the above manufacturing process.

[ evaluation test ]

The following evaluation test was carried out using a steel material and a forging heat treatment simulation.

[ coarse Al ₂ O ₃ Number density measurement test of inclusions]

Samples were taken from the R/2 portion (R is the radius connecting the surface of the steel material and the central axis) of the steel material (bar steel having a diameter of 40 mm) of each test number. Out of the surfaces of the samples, 30 samples each having a length of 4mm × a width of 2.5mm as a test area were sampled from a surface corresponding to a cross section (longitudinal section) including the axial direction of the steel material. Coarse Al was determined by the above method ₂ O ₃ Number density of inclusions/mm ² ). The determined coarse Al ₂ O ₃ Number density of inclusions/mm ² ) Number Density (pieces/mm) shown in Table 2 ² ) "one column.

[ microscopic Structure Observation ]

The forging heat treatment simulation of each test number was used to perform a microstructure observation test. Specifically, a sample including an R/2 portion was taken in the longitudinal section of the forging heat treatment simulation, and the sum (%) of the area ratios of tempered martensite and tempered bainite was determined by the above method. The sum (%) of the area ratios of tempered martensite and tempered bainite obtained is 90 to 100% and is referred to as "a" for evaluation, 85% or more and less than 90% as "B" for evaluation, 80% or more and less than 85% as "C" for evaluation, and less than 80% as "E" for evaluation. The evaluation results are shown in the column "microstructure" of table 2. In the case of the evaluations "a" to "C", the microstructure was judged to be mainly tempered martensite and/or tempered bainite, and in the case of the evaluation "E", the microstructure was judged not to be mainly tempered martensite and/or tempered bainite.

[ evaluation of Hot workability ]

Using the above method, 50 forging heat treatment simulants were produced for each test number. The presence or absence of cracks on the surface of the forged heat treatment simulation product after production was visually confirmed. The case where 0 out of 50 cracks occurred was designated as evaluation "a", the case where 1 crack occurred was designated as evaluation "B", the case where 2 to 3 cracks occurred was designated as evaluation "C", and the case where 4 or more cracks occurred was designated as evaluation "E". The evaluation results are shown in the column "hot workability" in table 2. When the evaluation values "a" to "C" were obtained, it was determined that sufficient hot workability was obtained, and when the evaluation value "E" was obtained, it was determined that the hot workability was low.

[ evaluation of yield Strength ]

2 test pieces of JIS 14A were sampled from a portion (inner region) other than a region (surface layer region) up to a depth position of 5mm from the surface of the forging heat treatment simulation of each test number. Using the collected test pieces, a tensile test was carried out at room temperature (25 ℃) in the air in accordance with JIS Z2241 (2011), and the average yield strength (MPa) of 2 specimens was obtained.

The yield strength (MPa) of 1500 to 1401MPa, 1400 to 1201MPa, 1200 to 1001MPa, and 1000 to 751MPa are referred to as "S" and "A" respectively, and "B" respectively. The case where the yield strength was 750MPa or less was referred to as evaluation "E". The evaluation results are shown in the column "yield strength" in table 2. When the evaluation results were "S" and "a" to "C", it was determined that sufficient yield strength could be obtained. In the case of the evaluation "E", it was judged that the yield strength was low.

[ fatigue Strength evaluation ]

From the region excluding the position cut off to a depth of 5mm from the surface of the forging heat treatment simulant of each test numberThe test piece of JIS 14A was used for the portion (inner region) other than the region (surface region). Using the collected test piece, an alternating stress fatigue test was carried out in accordance with JIS Z2273 (1978) at room temperature (25 ℃) in the atmosphere with a sine wave and a phase of 0 (MPa). Will repeat for 10 times ⁷ The maximum stress at which fracture does not occur next time is referred to as the fatigue strength (MPa). The frequency was set to 15Hz.

The fatigue strength (MPa) of 700 to 651MPa is referred to as "S" for evaluation, 650 to 601MPa is referred to as "A" for evaluation, 600 to 551MPa is referred to as "B" for evaluation, and 550 to 501MPa is referred to as "C" for evaluation. The fatigue strength of 500MPa or less was designated as "E". The evaluation results are shown in the column "fatigue strength" in table 2. When "S" and "a" to "C" were evaluated, it was determined that sufficient fatigue strength could be obtained. In the case of the evaluation "E", it was judged that the fatigue strength was low.

[ machinability evaluation ]

For each test number, 5 forging heat treatment simulants were prepared. Drill drilling was performed in the thickness direction at an arbitrary position on the prepared 5 forging heat treatment simulants, and the cutting resistance in the drill axial direction at the time of drill drilling was measured. The drill diameter was set to 8mm, and the rotation speed of the spindle was set to 720 rpm.

The case of cutting resistance of 1000 to 1099N was referred to as "S" for evaluation, the case of 1100 to 1199N was referred to as "A" for evaluation, the case of 1200 to 1299N was referred to as "B" for evaluation, and the case of 1300 to 1399N was referred to as "C" for evaluation. The cutting resistance of 1400N or more was designated as "E". The evaluation results are shown in the column "machinability" in table 2. When the evaluation results were "S" and "a" to "C", it was determined that sufficient machinability could be obtained. In the case of the evaluation "E", the machinability was judged to be low.

[ evaluation of cracking Properties ]

A test piece 10 simulating the large end portion 100 of the connecting rod 1 shown in fig. 2A was produced from the forging heat treatment simulation of each test number by machining. The test piece 10 had a length of 80mm on one side and a thickness of 10mm. A hole (through hole) 11 is formed in the center of the test piece 10. The hole 11 has a diameter of 60mm and its center is coaxial with the center of the test piece 10. As shown in fig. 2A, V-shaped notches M are formed in two places corresponding to the respective end points of the diameter in the outer edge of the hole 11. The depth of the notch M is 1mm, the curvature radius of the front end is 0.1mm, and the opening angle is 60 degrees.

The jig 12 is inserted into the hole 11. The jig 12 is formed of a pair of members in the shape of a half disc, and when the jig and the members are combined, a disc having a diameter D0 corresponding to the inner diameter of the hole 11 is formed. A hole 14 (see fig. 2B) for driving the wedge 13 is formed in the center of the jig 12.

After the jig 12 is fitted into the hole 11, a wedge 13 (see fig. 2B) is driven to break the test piece 10 at room temperature (25 ℃) into 2

pieces

10A and 10B (see fig. 2C).

Bolt hole processing is performed in the vicinity of both side surfaces of the

members

10A and 10B, and the

members

10A and 10B are fastened with bolts shown in fig. 2D. The maximum value Dmax and the minimum value Dmin of the diameter of the hole 11 of the test piece 10 after the fracture separation and after the bolt fastening were measured (see fig. 2D), and the difference therebetween was defined as the difference in the inside diameter deviation Δ D (= Dmax-Dmin, in μm).

The case where the inner diameter deviation Δ D is 0 to 10 μm is referred to as "A", the case where the inner diameter deviation Δ D is 11 to 20 μm is referred to as "B", the case where the inner diameter deviation Δ D is 21 to 30 μm is referred to as "C", and the case where the inner diameter deviation Δ D is 31 to 40 μm is referred to as "D". Further, the case where the inner diameter deformation amount Δ D exceeded 40 μm was referred to as evaluation "E". The evaluation results are shown in the column "Δ D" of table 2. When the "a" to "D" were evaluated, it was judged that sufficient cracking property could be obtained. When "E" was evaluated, it was judged that the cleavage property was low.

[ evaluation results ]

Referring to tables 1 to 2, the chemical compositions of test numbers E-1 to E-45 and C-13 to C-18 are suitable and satisfy formula (1). Further, the ladle, the aluminum deoxidizer, the addition rate of the deoxidizer, the timing of Si addition, and the holding time of the molten steel at 1600 ℃ or more are also suitable. As a result, coarse Al in the steel ₂ O ₃ The number density of the inclusions is 0.05 to 1.00 pieces/mm ² In the presence of a surfactant. As a result, the steel material can have excellent hot workability. As a result, the heat-treated product is forgedThe microstructure is further a tempered martensite and/or tempered bainite host, but still achieves excellent yield strength, excellent fatigue strength, excellent machinability and excellent fracture properties.

On the other hand, in test No. C-1, the Al content was too high. As a result, coarse Al ₂ O ₃ The number density of the inclusions exceeds 1.00/mm ² . As a result, hot workability of the steel material cannot be sufficiently obtained. As a result, the fatigue strength of the forged heat-treated product cannot be sufficiently obtained.

In test No. C-2, the Al content was too low. As a result, coarse Al ₂ O ₃ The number density of the inclusions is less than 0.05 pieces/mm ² . As a result, the cracking properties of the forged heat-treated product cannot be sufficiently obtained.

In test No. C-3, fn1 was too high. As a result, the fatigue strength of the forged heat-treated product cannot be sufficiently obtained.

In test No. C-4, fn1 was too low. As a result, the microstructure of the forged heat-treated product does not become a main component of tempered martensite and/or tempered bainite. As a result, the yield strength of the forged heat-treated product cannot be sufficiently obtained. As a result, the fatigue strength of the forged heat-treated product cannot be sufficiently obtained.

In test No. C-5, the chemical composition corresponds to example 19 of patent document 5. In test No. C-5, the Mn content was too high. Furthermore, fn1 is too high. Further, the time for holding the molten steel at 1600 ℃ or higher until the casting is started after the aluminum deoxidizer is added to the molten steel immediately after the tapping is too short. As a result, coarse Al ₂ O ₃ The number density of the inclusions is less than 0.05 pieces/mm ² . As a result, hot workability of the steel material cannot be sufficiently obtained. As a result, the fatigue strength of the forged heat-treated product cannot be sufficiently obtained. As a result, the cracking properties of the forged heat-treated product cannot be sufficiently obtained.

In test No. C-6, the ladle did not satisfy the conditions. As a result, coarse Al ₂ O ₃ The number density of the inclusions is less than 0.05 pieces/mm ² . As a result, the cracking properties of the forged heat-treated product cannot be sufficiently obtained.

In test No. C-7, the aluminum deoxidizer did not satisfy the conditions. As a result, coarse Al ₂ O ₃ The number density of the inclusions is less than 0.05 pieces/mm ² . As a result, the cracking properties of the forged heat-treated product cannot be sufficiently obtained.

In test No. C-8, the timing of Si addition did not satisfy the conditions. As a result, coarse Al ₂ O ₃ The number density of the inclusions is less than 0.05 pieces/mm ² . As a result, the cracking properties of the forged heat-treated product cannot be sufficiently obtained.

In test No. C-9, the addition rate of the additional deoxidizer was too high during the vacuum degassing treatment. As a result, coarse Al ₂ O ₃ The number density of the inclusions exceeds 1.00/mm ² . As a result, hot workability of the steel material cannot be sufficiently obtained. As a result, the fatigue strength of the forged heat-treated product cannot be sufficiently obtained.

In test No. C-10, the addition rate of the deoxidizer added during the vacuum degassing treatment was too low. As a result, coarse Al ₂ O ₃ The number density of the inclusions is less than 0.05 piece/mm ² . As a result, the cracking property of the forged heat-treated product cannot be sufficiently obtained.

In test No. C-11, the retention time of the molten steel at 1600 ℃ or higher was too long from the addition of the aluminum deoxidizer to the molten steel immediately after tapping until the start of casting. As a result, coarse Al ₂ O ₃ The number density of the inclusions exceeds 1.00/mm ² . As a result, hot workability of the steel material cannot be sufficiently obtained. As a result, the fatigue strength of the forged heat-treated product cannot be sufficiently obtained.

In test No. C-12, the retention time of the molten steel at 1600 ℃ or more was too short from the addition of the aluminum deoxidizer to the molten steel immediately after the tapping until the start of casting. As a result, coarse Al ₂ O ₃ The number density of the inclusions is less than 0.05 pieces/mm ² . As a result, the cracking properties of the forged heat-treated product cannot be sufficiently obtained.

[ example 2]

In example 2, a steel material having a Cr content of 0.51 to 2.50% was examined. Specifically, molten steels having chemical compositions shown in table 3 were produced. In table 3, "-" indicates that the content of the corresponding element is at an impurity level.

[ Table 3]

TABLE 3

Referring to Table 3, the chemical compositions of test Nos. E-46 to E-89 and C-24 to C-36 were suitable and satisfied formula (2). On the other hand, the chemical compositions of test Nos. C-19 to C-23 did not satisfy or did not satisfy formula (2). The chemical composition of test No. C-23 is within the range of the chemical composition of the steel described in patent document 6.

The molten steel of each test number was subjected to primary refining in a 70-ton converter in the same manner as in example 1, and was tapped into a ladle. Immediately after tapping to a ladle, an aluminum deoxidizer was added to perform deoxidation treatment in the same manner as in example 1. Similarly to example 1, the production conditions until the deoxidation treatment were carried out were shown in table 4 in the column "exclusive package", the column "deoxidizer Al ratio", the column "additional deoxidizer", the column "retention time at 1600 ℃ or higher", and the column "Si addition". The evaluation criteria in table 4 for "exclusive use package", additional deoxidizer ", retention time at 1600 ℃ or higher" and Si addition "are the same as in example 1.

[ Table 4]

TABLE 4

Next, as in example 1, a cast slab (bloom) was produced from the molten steel of each test number, the produced bloom was hot-rolled to produce a bar-shaped billet, and then, finish rolling was performed using a finish rolling mill to produce a bar steel having a diameter of 40 mm. By the above-described manufacturing process, a steel material (steel bar) is manufactured.

[ production of forged Heat treatment simulant ]

As in example 1, a sample material having a diameter of 40mm and a length of 100mm was sampled from a steel material (steel bar). The heating temperature at the time of hot forging, in which the test materials of the respective test numbers were held for 5 minutes, is shown in the column of "forging heating temperature (. Degree. C.)" in the same manner as in example 1. After heating, the resulting material was rapidly subjected to 90% thermal compression along the axial direction, molded into a disk shape, immersed in oil at 50 to 150 ℃ and cooled to 100 ℃ or lower, as in example 1. The temperature at which cooling is started is 800 ℃ or higher. In Table 4, the cooling rate of the test material for each test number is shown in the column "quenching cooling rate (. Degree. C./sec)". The cooling rate is determined based on the temperature measured at the center of the sample.

Similarly to example 1, the test materials of the respective test numbers after cooling were reheated and held for 30 minutes for tempering. The tempering temperature (. Degree. C.) of the test materials of each test number is shown in the column of "tempering temperature (. Degree. C.)". The tempering temperature and the tempering time are defined as in example 1. The forging heat treatment dummy is manufactured by the manufacturing process.

[ evaluation test ]

[ coarse Al ₂ O ₃ Number density measurement test of inclusions]

As in example 1, samples were taken from the R/2 part of the steel materials (steel bars having a diameter of 40 mm) of the respective test numbers. Of the surfaces of the samples, 30 samples each having a length of 4mm × a width of 2.5mm were sampled from the surface corresponding to a cross section (longitudinal section) including the axial direction of the steel material. Coarse Al was determined by the above method ₂ O ₃ Number density of inclusions/mm ² ). The obtained coarse Al ₂ O ₃ Number density of inclusions/mm ² ) The number density (number/mm) shown in Table 4 ² ) "one column.

[ microscopic Structure Observation ]

The forging heat treatment simulation of each test number was used to perform a microstructure observation test in the same manner as in example 1. Specifically, a sample including an R/2 portion was taken in the longitudinal section of the forging heat treatment simulation, and the sum (%) of the area ratios of tempered martensite and tempered bainite was determined by the above method. The sum (%) of the area ratios of tempered martensite and tempered bainite obtained is 90 to 100% and is referred to as "a" for evaluation, 85% or more and less than 90% as "B" for evaluation, 80% or more and less than 85% as "C" for evaluation, and less than 80% as "E" for evaluation. The evaluation results are shown in the column "microstructure" in table 4. In the case of the evaluations "a" to "C", the microstructure was judged to be mainly tempered martensite and/or tempered bainite, and in the case of the evaluation "E", the microstructure was judged not to be mainly tempered martensite and/or tempered bainite.

[ evaluation of Hot workability ]

In the same manner as in example 1, 50 forging heat treatment simulants were produced for each test number by the above-described method. The presence or absence of cracks on the surface of the forged heat treatment simulation product after production was visually confirmed. The case where 0 out of 50 cracks occurred was designated as "a", the case where 1 crack occurred was designated as "B", the case where 2 to 3 cracks occurred was designated as "C", and the case where 4 or more cracks occurred was designated as "E". The evaluation results are shown in the column "hot workability" in table 4. In the case of the evaluations "a" to "C", it was judged that sufficient hot workability could be obtained, and in the case of the evaluation "E", it was judged that the hot workability was low.

[ evaluation of yield Strength ]

In the same manner as in example 1, 2 test pieces of JIS 14A were sampled from the inner region of the forged heat-treated model of each test No. Using the collected test pieces, a tensile test was carried out at room temperature (25 ℃) in the air in accordance with JIS Z2241 (2011), and the average yield strength (MPa) of 2 specimens was obtained.

The case of yield strength (MPa) of 1600 to 1501MPa is referred to as "S" evaluation, the case of 1500 to 1301MPa is referred to as "A" evaluation, the case of 1300 to 1101MPa is referred to as "B" evaluation, and the case of 1100 to 851MPa is referred to as "C" evaluation. The case where the yield strength is 850MPa or less is referred to as evaluation "E". The evaluation results are shown in the column "yield strength" in table 4. When the evaluation results were "S" and "a" to "C", it was determined that sufficient yield strength could be obtained. In the case of the evaluation "E", it was judged that the yield strength was low.

[ fatigue Strength evaluation ]

In the same manner as in example 1, JIS 14A test piece was sampled from the inner region of the forged heat treatment simulation of each test number. Using the collected test piece, an alternating stress fatigue test was carried out in accordance with JIS Z2273 (1978) at room temperature (25 ℃) in the atmosphere with a sine wave and a phase of 0 (MPa). Will repeat for 10 times ⁷ The maximum stress at which fracture does not occur next time is referred to as the fatigue strength (MPa). The frequency was set to 15Hz.

The fatigue strength (MPa) of 750 to 701MPa is referred to as "S" for evaluation, the fatigue strength (MPa) of 700 to 651MPa is referred to as "A" for evaluation, the fatigue strength (MPa) of 650 to 601MPa is referred to as "B" for evaluation, and the fatigue strength (MPa) of 600 to 551MPa is referred to as "C" for evaluation. The fatigue strength was 550MPa or less and was referred to as "E". The evaluation results are shown in the column "fatigue strength" in table 4. When the evaluation results were "S" and "a" to "C", it was determined that sufficient fatigue strength could be obtained. In the case of the evaluation "E", it was judged that the fatigue strength was low.

[ machinability evaluation ]

As in example 1, drill drilling was performed in the thickness direction at arbitrary positions for 5 forging heat treatment specimens for each test number, and the cutting resistance in the drill axial direction during drill drilling was measured. The drill diameter was set to 8mm, and the rotation speed of the spindle was set to 720 rpm.

The case of cutting resistance of 1000 to 1099N was referred to as "S" for evaluation, the case of 1100 to 1199N was referred to as "A" for evaluation, the case of 1200 to 1299N was referred to as "B" for evaluation, and the case of 1300 to 1399N was referred to as "C" for evaluation. The cutting resistance of 1400N or more was designated as "E". The evaluation results are shown in the section "machinability" in table 4. When "S" and "a" to "C" were evaluated, it was determined that sufficient machinability could be obtained. In the case of the evaluation "E", the machinability was judged to be low.

[ evaluation of cracking Properties ]

A test piece 10 simulating the large end portion 100 of the connecting rod 1 shown in fig. 2A was produced from the forging heat treatment simulation of each test number by machining in the same manner as in example 1. The hole 11 formed in the center of the test piece 10 had a diameter of 60mm, and the center thereof was coaxial with the center of the test piece 10, as in example 1. In the same manner as in example 1, V-shaped notches M were formed in two places corresponding to the respective end points of the diameter in the outer edge of the hole 11. The depth of the notch M is 1mm, the curvature radius of the front end is 0.1mm, and the opening angle is 60 degrees.

After the jig 12 was fitted into the hole 11 in the same manner as in example 1, the wedge 13 (see fig. 2B) was driven, and the test piece 10 was broken and separated into 2

pieces

10A and 10B at room temperature (25 ℃) (see fig. 2C). The maximum value Dmax and the minimum value Dmin of the diameter of the hole 11 of the test piece 10 after the fracture separation and after the fastening of the bolt 15 were measured in the same manner as in example 1 (see fig. 2D), and the difference therebetween was defined as the inside diameter deviation difference Δ D (= Dmax-Dmin, in μm).

The case where the inner diameter deviation Δ D was 0 to 10 μm was referred to as "A" evaluation, "the case where the inner diameter deviation Δ D was 11 to 20 μm was referred to as" B "evaluation," the case where the inner diameter deviation Δ D was 21 to 30 μm was referred to as "C" evaluation, and the case where the inner diameter deviation Δ D was 31 to 40 μm was referred to as "D" evaluation. Further, the case where the inner diameter deformation amount Δ D exceeded 40 μm was referred to as evaluation "E". The evaluation results are shown in the column "Δ D" in table 4. When the "a" to "D" were evaluated, it was judged that the cleavage property could be obtained. When "E" was evaluated, it was judged that the cleavage property was low.

[ evaluation results ]

Referring to tables 3 to 4, the chemical compositions of test Nos. E-46 to E-89 and C-31 to C-36 were appropriate and satisfied formula (2). Further, the ladle, the aluminum deoxidizer, the addition rate of the deoxidizer, the timing of Si addition, and the holding time of the molten steel at 1600 ℃ or more are also suitable. As a result, coarse Al in the steel ₂ O ₃ The number density of the inclusions is 0.05 to 1.00 pieces/mm ² Within the range of (1). As a result, the steel material can have excellent hot workability. As a result, the microstructure of the forged heat-treated product is further tempered martensite and/or tempered BellevilleA bulk body, but still obtaining excellent yield strength, excellent fatigue strength, excellent machinability and excellent fracture properties.

On the other hand, in test No. C-19, the Al content was too high. As a result, coarse Al ₂ O ₃ The number density of the inclusions exceeds 1.00/mm ² . As a result, hot workability of the steel material cannot be sufficiently obtained. As a result, the fatigue strength of the forged heat-treated product cannot be sufficiently obtained.

In test No. C-20, the Al content was too low. As a result, coarse Al ₂ O ₃ The number density of the inclusions is less than 0.05 pieces/mm ² . As a result, the cracking properties of the forged heat-treated product cannot be sufficiently obtained.

In test No. C-21, fn1 was too high. As a result, the fatigue strength of the forged heat-treated product cannot be sufficiently obtained.

In test No. C-22, fn1 was too low. As a result, the microstructure of the forged heat-treated product does not form a main body of tempered martensite and/or tempered bainite. As a result, the yield strength of the forged heat-treated product cannot be sufficiently obtained. As a result, the fatigue strength of the forged heat-treated product cannot be sufficiently obtained.

In test No. C-23, the chemical composition corresponds to example 2 of patent document 6. In test No. C-23, the Si content was too low. Further, the ladle does not satisfy the condition. As a result, coarse Al ₂ O ₃ The number density of the inclusions is less than 0.05 pieces/mm ² . As a result, the hot workability of the steel material for hot forging cannot be sufficiently obtained. As a result, the yield strength of the forged heat-treated product cannot be sufficiently obtained. As a result, the fatigue strength of the forged heat-treated product cannot be sufficiently obtained. As a result, the machinability of the forged heat-treated product cannot be sufficiently obtained. As a result, the cracking properties of the forged heat-treated product cannot be sufficiently obtained. In test No. C-23, the heating temperature during hot forging did not satisfy the preferable range.

In test No. C-24, the ladle did not satisfy the conditions. As a result, coarse Al ₂ O ₃ The number density of the inclusions is less than 0.05 pieces/mm ² . As a result, sufficient forging cannot be obtainedCracking property of the heat-treated product.

In test No. C-25, the aluminum deoxidizer did not satisfy the conditions. As a result, coarse Al ₂ O ₃ The number density of the inclusions is less than 0.05 piece/mm ² . As a result, the cracking property of the forged heat-treated product cannot be sufficiently obtained.

In test No. C-26, the timing of Si addition did not satisfy the conditions. As a result, coarse Al ₂ O ₃ The number density of the inclusions is less than 0.05 piece/mm ² . As a result, the cracking properties of the forged heat-treated product cannot be sufficiently obtained.

In test No. C-27, the addition rate of the deoxidizer added during the vacuum degassing treatment was too high. As a result, coarse Al ₂ O ₃ The number density of the inclusions exceeds 1.00/mm ² . As a result, hot workability of the steel material cannot be sufficiently obtained. As a result, the fatigue strength of the forged heat-treated product cannot be sufficiently obtained.

In test No. C-28, the addition rate of the additional deoxidizer was too low during the vacuum degassing treatment. As a result, coarse Al ₂ O ₃ The number density of the inclusions is less than 0.05 pieces/mm ² . As a result, the cracking properties of the forged heat-treated product cannot be sufficiently obtained.

In test No. C-29, the molten steel was kept at 1600 ℃ or higher for an excessively long period of time from the addition of the aluminum deoxidizer to the molten steel immediately after tapping to the start of casting. As a result, coarse Al ₂ O ₃ The number density of the inclusions exceeds 1.00/mm ² . As a result, hot workability of the steel material cannot be sufficiently obtained. As a result, the fatigue strength of the forged heat-treated product cannot be sufficiently obtained.

In test No. C-30, the retention time of the molten steel at 1600 ℃ or more was too short from the time of adding the aluminum deoxidizer to the molten steel immediately after tapping to the time of starting casting. As a result, coarse Al ₂ O ₃ The number density of the inclusions is less than 0.05 piece/mm ² . As a result, the cracking property of the forged heat-treated product cannot be sufficiently obtained.

The embodiments of the present invention have been described above. However, the above embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above embodiment, and the above embodiment can be implemented by appropriately changing the embodiment within the scope not departing from the gist thereof.

Claims

1. A forged heat-treated product having the following chemical composition:

in mass percent

C：0.10～0.60％、

Si：0.05～1.00％、

Mn：0.30～1.50％、

P: less than 0.1000%,

S: less than 0.3000 percent,

Al：0.003～0.100％、

N: less than 0.0200%,

Cr：0～2.50％、

Cu：0～0.60％、

Ni：0～0.60％、

Mo：0～0.70％、

V：0～0.049％、

Ti：0～0.250％、

B：0～0.0050％、

Nb：0～0.100％、

Te：0～0.3000％、

Ca：0～0.0100％、

Bi:0 to 0.4000%, and

satisfying the formula (1) when the Cr content is 0-0.50%,

the Cr content is 0.51-2.50%, and the formula (2) is satisfied;

In the microstructure of the forged heat-treated product, the sum of the area ratios of tempered martensite and tempered bainite is 80% or more,

here, the symbol of the element in the formula (1) and the formula (2) is substituted with the content of the corresponding element in mass%, and when the corresponding element is not contained, the symbol of the element is substituted with "0", and fB in the formula (1) and the formula (2) is "0" when the content of B in mass% is 0% and "1" when the content of B in mass% exceeds 0%.

2. The forged heat-treated product according to claim 1,

Cr：0.01～2.50％、

Cu：0.01～0.60％、

Ni：0.01～0.60％、

Mo：0.01～0.70％、

V：0.005～0.049％、

Ti：0.005～0.250％、

B:0.0005 to 0.0050%, and

Nb：0.005～0.100％

1 or 2 or more of the group.

3. The forged heat-treated product as claimed in claim 1 or 2,

said chemical composition comprises a chemical composition selected from the group consisting of

Te：0.0003～0.3000％、

Ca:0.0003 to 0.0100%, and

Bi：0.0003～0.4000％

1 or 2 or more of the group.

4. A method for producing a forged heat-treated product, comprising the steps of:

a tempering step of holding the intermediate product at 400 to 650 ℃ for 30 to 90 minutes after the quenching step,

the steel material has the following chemical composition:

in mass%)

C：0.10～0.60％、

Si：0.05～1.00％、

Mn：0.30～1.50％、

P: less than 0.1000 percent,

S: less than 0.3000 percent,

Al：0.003～0.100％、

N: less than 0.0200%,

Cr：0～2.50％、

Cu：0～0.60％、

Ni：0～0.60％、

Mo：0～0.70％、

V：0～0.049％、

Ti：0～0.250％、

B：0～0.0050％、

Nb：0～0.100％、

Te：0～0.3000％、

Ca：0～0.0100％、

Bi:0 to 0.4000%, and

and the balance: fe and impurities in the iron-based alloy, wherein the impurities are,

the Cr content is 0-0.50%, the formula (1) is satisfied,

satisfying the formula (2) when the Cr content is 0.51-2.50%;

the coarse Al in the steel ₂ O ₃ The number density of the inclusions is 0.05 to 1.00 pieces/mm ² ，